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Completing the square

Completing the square is an algebraic technique for solving quadratic equations of the form ax^2 + bx + c = 0 by transforming the quadratic expression into a perfect square trinomial plus or minus a constant, allowing the use of the square root property to find the roots.^[1] This method dates back to ancient civilizations, with evidence of geometric approaches to completing squares appearing in Old Babylonian tablets around 1900 BC, where problems involving areas were solved by filling L-shaped regions to form squares.^[2] Later, in the 9th century, the Persian mathematician Al-Khwarizmi formalized the process in his treatise Al-Kitab al-mukhtasar fi hisab al-jabr wa-l-muqabala, using geometric constructions to handle cases of quadratic equations without negative numbers.^[2] The primary purpose of completing the square is to rewrite a quadratic polynomial ax^2 + bx + c in vertex form a(x - h)^2 + k, where the vertex of the associated parabola is at (h, k) with h = -b/(2a) and k = c - b^2/(4a).^[3] This form facilitates graphing parabolas, evaluating definite integrals involving quadratic expressions, and computing Laplace transforms in more advanced mathematics.^[3] Although less commonly used for direct equation solving due to its procedural complexity compared to the quadratic formula, it serves as the foundational derivation for that formula by applying the steps to the general quadratic equation.^[1] To complete the square, first ensure the leading coefficient a = 1 by dividing the equation by a if necessary; for example, in $2x^2 + 6x + 7 = 0, divide by 2 to get x^2 + 3x + 3.5 = 0. Move the constant term to the other side, then add \left(\frac{b}{2}\right)^2 (half the coefficient of x, squared) to both sides to form the perfect square: for x^2 + 4x = -3, add $4 to both sides yielding (x + 2)^2 = 1, which solves as x + 2 = \pm 1 or x = -1, -3.^[3] For non-monic quadratics, factor out a from the x^2 and x terms before completing the square inside the parentheses.^[3] This method is particularly efficient when the coefficient of the linear term is even, ensuring integer values during the process.^[4]

Introduction

Definition and purpose

Completing the square is an algebraic technique used to rewrite a quadratic polynomial of the form ax^2 + bx + c into its vertex form, a(x - h)^2 + k, where h and k are constants determined by the coefficients a, b, and c.^[3] This transformation involves manipulating the expression to form a perfect square trinomial, revealing the structural properties of the quadratic.^[5] The process assumes a basic understanding of quadratic polynomials and their standard form.^[6] The primary purpose of completing the square is to facilitate the solution of quadratic equations by converting them into a form that can be solved using square roots, thereby avoiding direct reliance on the quadratic formula.^[3] It also aids in graphing parabolas by directly identifying the vertex coordinates (h, k), which represent the turning point and axis of symmetry of the curve.^[5] Furthermore, this method provides insights into the minimum or maximum values of the quadratic function, occurring at the vertex: a minimum if a > 0 or a maximum if a < 0.^[6]

Basic example

To illustrate the process of completing the square, consider the quadratic equation x^2 + 6x + 5 = 0. This monic quadratic provides a straightforward case to demonstrate the method's steps, which transform the equation into a form that reveals its roots and vertex.^[3] Begin by isolating the quadratic and linear terms on one side of the equation:

x^2 + 6x + 5 = 0 \implies x^2 + 6x = -5

Here, the constant term is moved to the right side to prepare for forming a perfect square trinomial on the left.^[3] Next, take half of the coefficient of x (which is 6, so half is 3) and square it to get 9. Add this value to both sides of the equation:

x^2 + 6x + 9 = -5 + 9 \implies x^2 + 6x + 9 = 4

The left side now factors as a perfect square:

(x + 3)^2 = 4

This step completes the square by adjusting the expression to match the form (x + \frac{b}{2})^2, where b = 6.^[3] To solve for x, take the square root of both sides, accounting for both positive and negative roots:

x + 3 = \pm \sqrt{4} \implies x + 3 = \pm 2

Subtract 3 from both sides to isolate x:

x = -3 + 2 = -1 \quad \text{or} \quad x = -3 - 2 = -5

The roots are x = -1 and x = -5. Additionally, the completed square form of the original equation is (x + 3)^2 - 4 = 0, which highlights the vertex at (-3, -4) in the related quadratic function y = x^2 + 6x + 5.^[3]

Historical Development

Origins in ancient mathematics

The earliest known methods resembling the completion of the square emerged in Babylonian mathematics around 2000 BCE, where scholars solved quadratic equations through geometric dissections depicted on clay tablets. These ancient Mesopotamians interpreted quadratic problems geometrically, often framing them as finding dimensions of rectangles or fields with given areas and side differences. For instance, a typical problem involved a rectangle whose area was 60 and whose length exceeded its width by 7, leading to a geometric construction where half the difference was used to form a square added to the area, effectively completing a larger square whose side gave the solution. This approach, preserved on Old Babylonian tablets, relied on cut-and-paste manipulations of areas rather than symbolic algebra, demonstrating an intuitive grasp of quadratic relationships through visual and proportional reasoning.^[7] In ancient Greece, Euclid formalized similar geometric techniques in his Elements around 300 BCE, particularly in applications of the Pythagorean theorem. Book I, Proposition 47 provides a proof by constructing squares on the sides of a right-angled triangle and rearranging parallelograms and triangles to equate areas, a process that geometrically completes squares to demonstrate that the square on the hypotenuse equals the sum of the squares on the other two sides. Euclid's method involved drawing lines parallel to the sides and equating composite figures, such as showing that certain parallelograms equal the squares on the legs, thereby balancing areas without algebraic notation. This rigorous geometric framework influenced subsequent Western mathematics, emphasizing deduction from axioms to verify quadratic equalities.^[8] By the 7th century CE, Indian mathematician Brahmagupta advanced quadratic solutions in his Brahmasphutasiddhanta (628 CE), introducing algebraic rules that echoed completion of the square for equations like ax^2 + c = y^2. He provided general formulas for positive and negative solutions, treating cases where the constant term is added or subtracted, and generated infinite sequences of integer solutions for indeterminate equations, such as deriving pairs like (1, 3) and (6, 17) from $8x^2 + 1 = y^2. While Brahmagupta's approach was more arithmetic than purely geometric, it resembled completing the square by isolating terms to form perfect squares, marking a shift toward symbolic manipulation in non-European traditions.^[9]

Contributions from medieval and early modern scholars

During the Islamic Golden Age, the Persian scholar Muhammad ibn Musa al-Khwarizmi made foundational contributions to algebra in his treatise Al-Kitāb al-mukhtaṣar fī ḥisāb al-jabr wa-l-muqābala (The Compendious Book on Calculation by Completion and Balancing), composed around 820 CE. In this work, al-Khwarizmi systematically classified quadratic equations into six types and introduced the method of al-jabr—meaning "completion"—to solve them by eliminating negative terms and balancing equations, a process equivalent to completing the square. This approach involved geometric constructions alongside verbal descriptions, laying the groundwork for algebraic manipulation and directly inspiring the modern term "algebra."^[10] Building on this tradition in the 11th century, the Persian mathematician Omar Khayyam advanced the geometric-algebraic synthesis in his Treatise on Demonstration of Problems of Algebra, completed around 1070 while in Samarkand. Khayyam extended al-Khwarizmi's methods by classifying higher-degree equations and solving cubic equations through intersections of conic sections, which provided rigorous geometric proofs for algebraic solutions. His work emphasized the limitations of radical solutions for certain equations, promoting a unified framework that integrated completing the square with broader algebraic problems.^[11] In Renaissance Europe, the adoption and refinement of these Islamic algebraic techniques occurred in the 16th century, notably through Italian and French scholars. Girolamo Cardano, in his 1545 publication Ars Magna, incorporated completing the square as a preliminary step in deriving radical solutions for cubic and quartic equations, adapting al-Khwarizmi's methods to handle negative roots and complex quantities for the first time. Similarly, François Viète advanced symbolic notation in works like In artem analyticam isagoge (1591), enabling more abstract applications of completion techniques to quadratics and higher degrees, which facilitated systematic equation transformations and influenced the shift toward modern algebraic symbolism.^[12]^[13]

Core Method

General procedure for monic quadratics

A monic quadratic equation is one where the coefficient of the x^2 term is 1, typically expressed as x^2 + bx + c = 0, with b and c as constants.^[3] The completing the square procedure rewrites this equation to reveal its roots by forming a perfect square trinomial on one side, facilitating solution via square roots.^[14] To begin, isolate the constant term by moving c to the right side, yielding x^2 + bx = -c. This step prepares the left side for adjustment into a perfect square while preserving equality.^[15] Next, add \left(\frac{b}{2}\right)^2 to both sides: x^2 + bx + \left(\frac{b}{2}\right)^2 = -c + \left(\frac{b}{2}\right)^2. This addition completes the square on the left, as the binomial expansion of \left(x + \frac{b}{2}\right)^2 = x^2 + b x + \left(\frac{b}{2}\right)^2 matches the original quadratic terms exactly; halving b works because the middle term in the expansion is $2 \cdot \frac{b}{2} \cdot x = b x, aligning the coefficients precisely.^[3]^[14] The left side now factors as \left(x + \frac{b}{2}\right)^2 = -c + \left(\frac{b}{2}\right)^2. To solve for x, take the square root of both sides: x + \frac{b}{2} = \pm \sqrt{-c + \left(\frac{b}{2}\right)^2}, then subtract \frac{b}{2} from both sides to isolate x. This yields the roots explicitly, provided the expression under the radical is non-negative for real solutions.^[15] In general, without setting the equation to zero, the monic quadratic x^2 + b x + c can be rewritten as \left(x + \frac{b}{2}\right)^2 + \left(c - \left(\frac{b}{2}\right)^2\right). This vertex form highlights the quadratic's minimum or maximum at x = -\frac{b}{2}, with the constant term adjusting for the original c.^[3] The rationale again stems from the binomial expansion, ensuring the transformation preserves the polynomial's value for all x.^[14]

Formula derivation

To derive the vertex form of a monic quadratic function f(x) = x^2 + bx + c, begin by considering the expanded form of the vertex form (x + h)^2 + k. Expanding this gives:

(x + h)^2 + k = x^2 + 2hx + h^2 + k.

^[5] Matching coefficients with the standard form x^2 + bx + c requires $2h = b, so h = \frac{b}{2}, and the constant term h^2 + k = c, yielding k = c - \left(\frac{b}{2}\right)^2 = c - \frac{b^2}{4}.^[16] For the full derivation via completing the square, start with x^2 + bx + c and focus on the quadratic and linear terms: x^2 + bx. To complete the square, add and subtract \left(\frac{b}{2}\right)^2:

x^2 + bx = x^2 + bx + \left(\frac{b}{2}\right)^2 - \left(\frac{b}{2}\right)^2 = \left(x + \frac{b}{2}\right)^2 - \frac{b^2}{4}.

^[5] Incorporating the constant term then gives:

x^2 + bx + c = \left(x + \frac{b}{2}\right)^2 - \frac{b^2}{4} + c = \left(x + \frac{b}{2}\right)^2 + \left(c - \frac{b^2}{4}\right).

^[16] This establishes the vertex form (x + h)^2 + k with h = \frac{b}{2} and k = c - \frac{b^2}{4}. To verify equivalence, expand the completed form back: \left(x + \frac{b}{2}\right)^2 + \left(c - \frac{b^2}{4}\right) = x^2 + bx + \frac{b^2}{4} + c - \frac{b^2}{4} = x^2 + bx + c.^[5] This confirms the algebraic identity holds for all b and c.^[16]

Extensions to Non-Standard Forms

Non-monic quadratic case

When the leading coefficient a of a quadratic equation ax^2 + bx + c = 0 is not equal to 1, the completing the square method requires an initial adjustment to transform the equation into a monic form before proceeding with the standard procedure.^[1] This step ensures the coefficient of the x^2 term is 1, allowing the familiar monic completing the square technique to be applied.^[1] The procedure begins by dividing the entire equation by a (assuming a \neq 0) to yield x^2 + \frac{b}{a}x + \frac{c}{a} = 0.^[1] Rearranging gives x^2 + \frac{b}{a}x = -\frac{c}{a}. To complete the square, add \left(\frac{b}{2a}\right)^2 = \frac{b^2}{4a^2} to both sides, resulting in \left(x + \frac{b}{2a}\right)^2 = -\frac{c}{a} + \frac{b^2}{4a^2} = \frac{b^2 - 4ac}{4a^2}.^[1] Multiplying through by a to return to the original scale produces the completed form a\left(x + \frac{b}{2a}\right)^2 + \left(c - \frac{b^2}{4a}\right) = 0.^[3] In expanded vertex form, the quadratic expression is ax^2 + bx + c = a\left(x + \frac{b}{2a}\right)^2 + \left(c - \frac{b^2}{4a}\right).^[3] For an illustrative example, consider $3x^2 - 12x + 7 = 0. Dividing by 3 gives x^2 - 4x + \frac{7}{3} = 0, or x^2 - 4x = -\frac{7}{3}. Adding \left(\frac{-4}{2}\right)^2 = 4 to both sides yields (x - 2)^2 = -\frac{7}{3} + 4 = \frac{5}{3}. Multiplying back by 3 results in $3(x - 2)^2 + \left(7 - \frac{(-12)^2}{4 \cdot 3}\right) = 3(x - 2)^2 - 5 = 0, confirming the form.^[1] The sign of a influences the geometric interpretation and root behavior. If a > 0, the parabola opens upward, with a minimum at the vertex; if a < 0, it opens downward, featuring a maximum.^[17] In the completed form a(x - h)^2 + k = 0 where h = -\frac{b}{2a} and k = c - \frac{b^2}{4a} = -\frac{D}{4a} with discriminant D = b^2 - 4ac, the nature of real roots depends on D: D > 0 implies two distinct real roots, D = 0 one real root (repeated), and D < 0 no real roots, regardless of the sign of a.^[6] This relation arises directly from the completed square, as solving requires the right side -\frac{k}{a} to be non-negative for real solutions.^[6] When a < 0, the procedure may alternatively involve factoring out -|a| from the x^2 and x terms first to ensure a positive inner coefficient, though the division method yields equivalent results.^[17] For instance, in -2x^2 + 8x - 5 = 0, dividing by -2 gives x^2 - 4x + \frac{5}{2} = 0, leading to (x - 2)^2 = \frac{3}{2}, or in form -2(x - 2)^2 + 3 = 0. This highlights the downward-opening parabola and confirms D = 24 > 0 for two real roots.^[6]

Matrix completion of squares

In multivariable calculus and optimization, completing the square extends to quadratic forms, which are second-degree polynomials in vector variables of the form \mathbf{x}^T A \mathbf{x} + \mathbf{b}^T \mathbf{x} + c, where \mathbf{x} \in \mathbb{R}^n, A \in \mathbb{R}^{n \times n} is a symmetric matrix, \mathbf{b} \in \mathbb{R}^n is a vector, and c \in \mathbb{R} is a scalar.^[18] This representation captures interactions between variables through the off-diagonal elements of A.^[19] Assuming A is invertible, the quadratic form can be rewritten by completing the square as (\mathbf{x} + A^{-1} \mathbf{b}/2)^T A (\mathbf{x} + A^{-1} \mathbf{b}/2) + (c - \mathbf{b}^T A^{-1} \mathbf{b}/4).^[18] The linear term vanishes after the shift \mathbf{x}' = \mathbf{x} + A^{-1} \mathbf{b}/2, isolating the quadratic and constant terms.^[20] This transformation is derived algebraically by expanding the squared term and matching coefficients, analogous to the scalar case but using matrix inversion.^[18] Alternatively, it follows from properties of the Schur complement in the associated block matrix or from eigenvalue decomposition to diagonalize A, though the direct expansion suffices for the completion.^[21] If A is positive definite, the completed form attains a global minimum at \mathbf{x} = -A^{-1} \mathbf{b}/2, with the minimum value c - \mathbf{b}^T A^{-1} \mathbf{b}/4.^[19] For example, consider the quadratic form $2x^2 + 4xy + 2y^2, which has no linear or constant terms and corresponds to \mathbf{x}^T \begin{pmatrix} 2 & 2 \\ 2 & 2 \end{pmatrix} \mathbf{x} where \mathbf{x} = \begin{pmatrix} x \\ y \end{pmatrix}.^[22] This simplifies to $2(x + y)^2, a completed square expression equivalent to (\mathbf{x} + \mathbf{0})^T A (\mathbf{x} + \mathbf{0}) + 0, highlighting the rank-one structure of A and the absence of adjustment needed.^[22]

Applications in Equation Solving

Extracting roots from quadratics

Completing the square provides a direct method to extract the roots of a quadratic equation by transforming it into a perfect square form, from which the solutions can be obtained using the square root property. For a monic quadratic equation x^2 + bx + c = 0, after completing the square, the equation becomes (x + \frac{b}{2})^2 = \frac{b^2}{4} - c. The roots are then given by x = -\frac{b}{2} \pm \sqrt{\frac{b^2}{4} - c}, where the expression under the square root is the negative of the constant term adjusted for the completion process.^[23]^[1] For the general quadratic ax^2 + bx + c = 0 with a \neq 0, the process involves first dividing through by a to make the leading coefficient 1, then proceeding as above. This yields the completed square form (x + \frac{b}{2a})^2 = \frac{b^2 - 4ac}{4a^2}, and the roots are x = -\frac{b}{2a} \pm \frac{\sqrt{b^2 - 4ac}}{2a}. Here, b^2 - 4ac is the discriminant, which determines the nature of the roots: if it equals zero, there is a repeated root at x = -\frac{b}{2a}; if positive, two distinct real roots; if negative, the roots are complex (though this method focuses on real extraction).^[24]^[1] This approach offers advantages over directly applying the quadratic formula, as it avoids the need to memorize the expression x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} and instead derives the roots step-by-step through algebraic manipulation. It also visually emphasizes the repeated root case when the right side of the completed equation is zero, providing intuitive insight into the equation's solution structure.^[24]^[1] The roots obtained via completing the square are mathematically equivalent to those from the quadratic formula, as the latter is precisely derived by applying this completion process to the general quadratic. For instance, consider x^2 + 6x + 5 = 0: completing the square gives (x + 3)^2 = 4, so x = -3 \pm 2, yielding roots x = -1 and x = -5. This equivalence holds without altering the solution set, but the method reinforces understanding of the underlying algebra.^[23]^[1]

Handling irrational and complex roots

Completing the square provides a direct way to identify irrational roots in quadratic equations when the discriminant is positive but not a perfect square, resulting in square roots of non-square integers. For instance, starting with the equation x^2 - 10x + 18 = 0, move the constant term to obtain x^2 - 10x = -18, then add ( -10/2 )^2 = 25 to both sides: x^2 - 10x + 25 = 7, which factors as (x - 5)^2 = 7. Taking square roots yields x - 5 = \pm \sqrt{7}, so the roots are x = 5 \pm \sqrt{7}, both irrational numbers.^[3] This process highlights the irrationality without invoking the full quadratic formula, as the non-integer square root emerges naturally from the completed form.^[25] For cases where the discriminant is negative, completing the square reveals complex roots by leading to a square equal to a negative value, necessitating the imaginary unit i = \sqrt{-1}. Consider the equation x^2 + 4x + 5 = 0: rearrange to x^2 + 4x = -5, add (4/2)^2 = 4 to both sides: x^2 + 4x + 4 = -1, or (x + 2)^2 = -1. The solutions are x + 2 = \pm \sqrt{-1} = \pm i, hence x = -2 \pm i.^[3] Similarly, for x^2 + 1 = 0, it simplifies directly to x^2 = -1, so x = \pm i.^[25] This technique demonstrates that no real solutions exist while providing the exact complex roots.^[1] The ability of completing the square to uncover both irrational and complex roots underscores its role in revealing the full solution set of quadratic equations, independent of the quadratic formula. In particular, when real roots are absent, the method aligns with the fundamental theorem of algebra, which guarantees that every quadratic polynomial has two roots (counting multiplicity) in the complex numbers. This approach not only determines the nature of the roots—real irrational or non-real complex—but also ties directly to the algebraic structure ensuring solvability over the complexes.^[26]

Graphical and Geometric Interpretations

Relation to parabola graphs

Completing the square provides a method to rewrite a quadratic function from its standard form ax^2 + bx + c into the vertex form f(x) = a(x - h)^2 + k, which is particularly useful for graphing parabolas as it immediately identifies the vertex and other graphical properties.^[27] In this form, the coordinates of the vertex are (h, k), representing the highest or lowest point of the parabola depending on the sign of a.^[5] The axis of symmetry is the vertical line x = h, which divides the parabola into two mirror-image halves.^[28] Additionally, the value of a determines the direction of opening: if a > 0, the parabola opens upward, indicating a minimum at the vertex; if a < 0, it opens downward, indicating a maximum.^[27] The vertex form also highlights the transformations that shape the parabola relative to the parent function y = x^2. A horizontal shift occurs by h units to the right if h > 0 or to the left if h < 0, while a vertical shift by k units moves the graph up if k > 0 or down if k < 0.^[5] The coefficient a scales the graph vertically by a factor of |a|: values of |a| > 1 cause compression toward the x-axis, $0 < |a| < 1 cause stretching away from it, and a negative a reflects the parabola over the x-axis.^[29] These transformations allow for quick sketching by starting from the vertex and plotting symmetric points, such as those at x = h \pm 1, without needing to solve for intercepts initially.^[27] For example, consider the quadratic function f(x) = x^2 + 6x + 5. Completing the square transforms it to f(x) = (x + 3)^2 - 4, revealing a vertex at (-3, -4) and a = 1 > 0, so the parabola opens upward with its axis of symmetry at x = -3.^[28] This form shows a horizontal shift left by 3 units and a vertical shift down by 4 units from y = x^2, enabling efficient graphing by marking the vertex and adding points like f(-2) = -3 and f(-4) = -3.^[27]

Geometric construction

The geometric construction of completing the square originates from ancient mathematical practices, where quadratic expressions were visualized using areas of squares and rectangles rather than algebraic symbols. In this approach, the term x^2 + bx is represented as the area of a square with side length x adjacent to a rectangle with dimensions x by b. The constant term c is treated as an additional rectangular or square area to be incorporated. This visualization allows for a physical manipulation of shapes to reveal the structure of a perfect square.^[2] To complete the square, a smaller square with side length b/2 (area (b/2)^2) is added to the figure and simultaneously subtracted in the algebraic sense, transforming the original shape into a larger square with side length x + b/2. Visually, the initial square and rectangle form an incomplete L-shaped region, often called a gnomon in ancient geometry, which lacks the corner piece. Adding this missing square completes the L-shape into the full larger square, while the subtracted area adjusts for the constant term, leaving a remainder that equals c - (b/2)^2. For instance, in solving x^2 + 10x = 39, the rectangle of area 10x is halved along its length, and a 5 by 5 square (area 25) is added to form a square of side x + 5 with total area 64, yielding the side length 8 and solution x = 3. This method was employed in Old Babylonian tablets around 1900 BC, such as YBC 6967, where areas were manipulated similarly to solve quadratics.^[2]^[30] The construction ties directly to the Pythagorean theorem through the use of gnomons, the L-shaped regions that represent the difference in areas between consecutive squares. Algebraically, this manifests in the identity x^2 + 2hx = (x + h)^2 - h^2, where the left side is the area of a square plus an L-shaped gnomon of width $2h, and the right side subtracts the smaller square to form the larger one, mirroring geometric proofs of a^2 + b^2 = c^2 by successive gnomon additions. Such techniques were later formalized by mathematicians like al-Khwarizmi in the 9th century, who illustrated quadratic solutions with diagrams of squares and appended rectangles completed via added squares.^[31]^[32]

Further Applications

In calculus and integration

Completing the square provides a method to rewrite the indefinite integral of a quadratic polynomial in a form that highlights its vertex, facilitating substitution and evaluation. For the integral \int (ax^2 + bx + c) \, dx, first complete the square on the quadratic expression to obtain a(x - h)^2 + k, where h = -b/(2a) and k = c - b^2/(4a). This transforms the integral to a \int (x - h)^2 \, dx + k \int dx = \frac{a}{3}(x - h)^3 + kx + C. Using the substitution u = x - h, the integral simplifies further to \frac{a}{3}u^3 + k(u + h) + C, which can be expressed back in terms of x. Consider the example \int (x^2 + 4x + 3) \, dx. Completing the square gives x^2 + 4x + 3 = (x + 2)^2 - 1, so the integral becomes \int [(x + 2)^2 - 1] \, dx = \frac{1}{3}(x + 2)^3 - x + C. Another important application is in integrals involving exponentials of quadratic forms, such as the Gaussian integral. For \int_{-\infty}^{\infty} e^{-(a x^2 + b x + c)} \, dx with a > 0, completing the square in the exponent gives -(a x^2 + b x + c) = -a (x + \frac{b}{2a})^2 + \frac{b^2}{4a} - c. Thus, the integral is e^{\frac{b^2}{4a} - c} \int_{-\infty}^{\infty} e^{-a (x + \frac{b}{2a})^2} \, dx = e^{\frac{b^2}{4a} - c} \sqrt{\frac{\pi}{a}}, using the known result \int_{-\infty}^{\infty} e^{-a u^2} \, du = \sqrt{\frac{\pi}{a}}. This technique is essential for evaluating error functions and in probability for normal distributions. A primary application in integration arises when evaluating \int \frac{1}{ax^2 + bx + c} \, dx, where completing the square converts the denominator to a(u^2 + p) or a(u^2 - p^2), with u = x - h. If the discriminant b^2 - 4ac < 0, yielding u^2 + p^2 (p > 0), substitution leads to \frac{1}{a} \int \frac{1}{u^2 + p^2} \, du = \frac{1}{ap} \arctan\left(\frac{u}{p}\right) + C. If the discriminant > 0, it results in a logarithmic form \frac{1}{2ap} \ln \left| \frac{u - p}{u + p} \right| + C. This approach avoids partial fraction decomposition for irreducible quadratics, simplifying the process directly. In broader calculus contexts, completing the square aids in computing arc lengths of curves where the integrand involves \sqrt{1 + (y')^2} and y is quadratic, producing a \sqrt{\text{quadratic}} that benefits from the technique followed by trigonometric or hyperbolic substitutions. Such calculations are relevant in physics for determining the actual path length of parabolic trajectories, as in projectile motion.

In complex analysis and numbers

In the context of complex quadratic equations, completing the square provides a method to express the polynomial in a form that reveals its roots within the complex numbers, often as elements of the Gaussian integers \mathbb{Z}, the ring of integers in \mathbb{Q}(i). For a quadratic equation z^2 + bz + c = 0 with complex coefficients, the technique involves adding and subtracting (b/2)^2 to rewrite it as (z + b/2)^2 = (b/2)^2 - c, yielding roots z = -b/2 \pm \sqrt{(b/2)^2 - c}. If the coefficients b and c are Gaussian integers, the roots frequently lie in \mathbb{Z} as well, facilitating factorization in this unique factorization domain. A representative example is the equation z^2 + 2z + 2 = 0. Completing the square gives z^2 + 2z + 1 + 1 = 0, or (z + 1)^2 = -1, so z + 1 = \pm i and z = -1 \pm i, both Gaussian integers. This process not only solves the equation but also demonstrates how completing the square aligns with the algebraic structure of \mathbb{Z}, where the norm N(a + bi) = a^2 + b^2 aids in verifying irreducibility and factorization. In complex analysis, completing the square simplifies the evaluation of contour integrals via the residue theorem, particularly for integrals involving Gaussian-like exponents. For instance, to compute \int_{-\infty}^{\infty} e^{-x^2} \cos(2bx) \, dx for b > 0, consider the complex function f(z) = e^{-z^2 + 2ibz} over a semicircular contour in the upper half-plane. Completing the square in the exponent yields -z^2 + 2ibz = -(z - ib)^2 - b^2, allowing a shift of the contour to w = z - ib, where the integral reduces to e^{-b^2} \int_{-\infty}^{\infty} e^{-w^2} \, dw = \sqrt{\pi} e^{-b^2} by Cauchy's theorem, as the function is entire and the arc contribution vanishes. This technique factors the quadratic exponent over the complexes, enabling residue computations or contour deformations without poles.^[33] Furthermore, completing the square connects to the Eisenstein integers \mathbb{Z}[\omega], where \omega = e^{2\pi i / 3}, by aiding the analysis of quadratic forms and units essential for unique factorization. In \mathbb{Q}(\sqrt{-3}), the norm form N(a + b\omega) = a^2 - ab + b^2 leads to equations like (2a + b)^2 + 3b^2 = 4N(\epsilon) for units \epsilon, solved via completing the square to identify the six units \{\pm 1, \pm \omega, \pm \omega^2\}. This structural insight supports the proof that \mathbb{Z}[\omega] is a unique factorization domain, where non-unit elements factor uniquely into irreducibles up to units, with irreducibles classified by norms congruent to primes modulo 3.^[34]

Variations and Generalizations

Completing higher powers like cubes

To generalize the completing the square method to cubic equations of the form x^3 + ax^2 + bx + c = 0, the first step is to depress the cubic by substituting x = y - \frac{a}{3}, which eliminates the quadratic term and yields a depressed cubic y^3 + py + q = 0, where p = b - \frac{a^2}{3} and q = c - \frac{ab}{3} + \frac{2a^3}{27}.^[35] This transformation simplifies the equation but does not directly allow for a straightforward "completing the cube" analogous to the quadratic case.^[35] Attempts to complete the cube involve assuming a form like y^3 + py + q = (y + k)^3 + ry + s and solving for constants k, r, s, but this generally fails to simplify the equation fully for arbitrary p and q, as linear substitutions cannot transform it into a pure cube y^3 + C = 0.^[35] Instead, more advanced substitutions, such as y = w + \frac{p}{3w}, reduce the depressed cubic to a quadratic equation in w^3, which is the basis of Cardano's formula: the roots are given by

\sqrt{{grok:render&&&type=render_inline_citation&&&citation_id=3&&&citation_type=wikipedia}}{-\frac{q}{2} + \sqrt{\left(\frac{q}{2}\right)^2 + \left(\frac{p}{3}\right)^3}} + \sqrt{{grok:render&&&type=render_inline_citation&&&citation_id=3&&&citation_type=wikipedia}}{-\frac{q}{2} - \sqrt{\left(\frac{q}{2}\right)^2 + \left(\frac{p}{3}\right)^3}}

. This process can be viewed as a partial completion, where the cubic is expressed as a sum involving cube roots rather than a single perfect cube. For illustration, consider the polynomial x^3 - 3x^2 + 2x. This can be partially completed by noting that (x-1)^3 = x^3 - 3x^2 + 3x - 1, so rearranging gives x^3 - 3x^2 + 2x = (x-1)^3 - x + 1.^[35] Such partial decompositions highlight the method's utility for specific coefficients but do not extend generally. Unlike completing the square, no universal "completing the cube" exists for arbitrary cubics due to the invariants under substitution, such as a^2 - 3b remaining unchanged, preventing reduction to a perfect cube form.^[35] Historical efforts, including those by François Viète in the late 16th century, explored substitutions like y = z - \frac{p}{3z} for depressed cubics to facilitate solutions, motivated by problems involving sums and sums of cubes, but these led to resolvents rather than full completions.^[36] Completing the square extends to biquadratic equations of the form x^4 + a x^2 + b = 0, where the even powers allow substitution y = x^2 to reduce it to a quadratic y^2 + a y + b = 0.^[37] To solve, complete the square on the quadratic in y:

y^2 + a y + b = \left(y + \frac{a}{2}\right)^2 - \left(\frac{a}{2}\right)^2 + b = 0

\left(y + \frac{a}{2}\right)^2 = \frac{a^2}{4} - b

Assuming the right side is nonnegative for real solutions, take square roots:

y + \frac{a}{2} = \pm \sqrt{\frac{a^2}{4} - b}, \quad y = -\frac{a}{2} \pm \sqrt{\frac{a^2}{4} - b}

The roots for x are then x = \pm \sqrt{y} for each positive y, yielding up to four real or complex roots depending on the discriminant \frac{a^2}{4} - b. This approach leverages the quadratic completing-the-square method after substitution and is detailed in precalculus resources for handling such quartics.^[38] Another application arises in optimization, particularly for minimizing reciprocal sums like x + \frac{1}{x} where x > 0. Substitute t = \sqrt{x} (so t > 0) to rewrite the expression as t^2 + \frac{1}{t^2}. Completing the square gives:

t^2 + \frac{1}{t^2} = \left(t - \frac{1}{t}\right)^2 + 2 \geq 2,

since \left(t - \frac{1}{t}\right)^2 \geq 0, with equality when t = 1 (i.e., x = 1). Thus, the minimum value is 2. This technique provides an elementary proof of the two-term AM-GM inequality, as \frac{x + 1/x}{2} \geq \sqrt{x \cdot 1/x} = 1 implies x + 1/x \geq 2, with equality under the same condition, connecting algebraic manipulation to inequality bounds in optimization contexts.^[39]

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