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Section formula

The section formula in coordinate geometry is a fundamental mathematical tool used to calculate the coordinates of a point that divides the joining two given points, either internally or externally, in a specified m:n. This formula bridges algebraic expressions with geometric positions on a , enabling precise determination of point locations along a line. For internal division, where the point lies between the two endpoints A(x_1, y_1) and B(x_2, y_2), the coordinates of the dividing point P are given by
P\left( \frac{m x_2 + n x_1}{m + n}, \frac{m y_2 + n y_1}{m + n} \right). In contrast, for external division, where the point lies outside the segment, the coordinates are
P\left( \frac{m x_2 - n x_1}{m - n}, \frac{m y_2 - n y_1}{m - n} \right), assuming m \neq n. A special case is the formula, derived when m:n = 1:1, yielding P\left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right).
The section formula finds extensive applications in , such as computing centroids of triangles, verifying of points, and solving problems in vector geometry and for between coordinates. It also aids in determining the ratio in which a given point divides a , enhancing problem-solving in fields like physics for position vectors and for spatial calculations.

Two-dimensional geometry

Internal division

In coordinate geometry, internal division occurs when a point lies between the two endpoints of a and divides it in a specified m:n, where m and n are indicating the proportional segments from the division point to each endpoint. This concept allows for the precise location of points along a based on weighted proportions, forming a foundational tool in for solving problems involving positions and intersections. The coordinates of the point P that divides the joining points A(x_1, y_1) and B(x_2, y_2) internally in the m:n are given by the section formula: P\left( \frac{n x_1 + m x_2}{m + n}, \frac{n y_1 + m y_2}{m + n} \right). Here, the weights n and m reflect the relative distances, ensuring the point is positioned such that the segment from A to P is to the segment from P to B as m:n. This formula arises directly from the principles of coordinate systems in the . To derive the formula, consider the parametric representation along the . The position of P can be expressed as a weighted , where the coordinate is shifted from A by a \frac{m}{m+n} of the total displacement to B. For the x-coordinate: x = x_1 + \frac{m}{m + n} (x_2 - x_1) = x_1 + \frac{m x_2 - m x_1}{m + n} = \frac{(m + n) x_1 + m x_2 - m x_1}{m + n} = \frac{n x_1 + m x_2}{m + n}. The y-coordinate follows analogously: y = y_1 + \frac{m}{m + n} (y_2 - y_1) = \frac{n y_1 + m y_2}{m + n}. This derivation relies on the of distances in a straight line, confirming the through the uniform scaling of coordinates. A geometric proof using similar triangles reinforces this: construct horizontal lines from A and P to align with the y-level of B, and vertical lines to form triangles whose similarity (by AA criterion, sharing and proportional sides) yields the same m:n in both x- and y-directions, leading to the . For example, if A(1, 2) and B(3, 4) are divided internally in the $1:1, the coordinates of P are \left( \frac{1 \cdot 1 + 1 \cdot 3}{1 + 1}, \frac{1 \cdot 2 + 1 \cdot 4}{1 + 1} \right) = (2, 3), illustrating a balanced position. The origins of the section formula trace back to the development of by in his 1637 work , where coordinate methods enabled algebraic representation of geometric divisions; it gained prominence in 19th-century textbooks as a standard tool for plane geometry problems.

External division

External division refers to the case where a point divides the line segment joining two points in a given ratio but lies outside the segment, extending the line beyond one of the endpoints. In coordinate geometry, this occurs when the division ratio m:n results in the point P being positioned such that the directed segments satisfy AP:PB = m:n, but with P not between A and B. For points A(x₁, y₁) and B(x₂, y₂), the coordinates of the point P dividing AB externally in the ratio m:n are given by: P\left( \frac{n x_1 - m x_2}{n - m}, \frac{n y_1 - m y_2}{n - m} \right) This formula can equivalently be written as \left( \frac{m x_2 - n x_1}{m - n}, \frac{m y_2 - n y_1}{m - n} \right), reflecting the sign convention for directed distances. The derivation of this formula relies on the geometry of similar triangles and the concept of directed segments. Consider points A(x₁, y₁) and B(x₂, y₂) on the plane, with P(x, y) dividing AB externally in ratio m:n. Drop perpendiculars from A, B, and P to the x-axis, meeting at points C, D, and M respectively, forming right triangles AMC and BND. Since P is external, the triangles ∆AMC and ∆BND are similar because their corresponding angles are equal (both right-angled, and sharing the angle with the line AB). The ratio of similarity is m:n, so the ratios of corresponding sides are equal: \frac{AM}{BN} = \frac{m}{n}. Substituting the lengths, AM = |x - x₁| and BN = |x₂ - x|, but accounting for direction in external division (where P is beyond B, say), yields \frac{x - x_1}{x_2 - x} = -\frac{m}{n} due to the opposite orientation. Solving \frac{x - x_1}{x_2 - x} = -\frac{m}{n}: cross-multiplying gives n(x - x_1) = -m (x_2 - x). Expanding, n x - n x_1 = -m x_2 + m x, then n x - m x = n x_1 - m x_2, so x (n - m) = n x_1 - m x_2, hence x = \frac{n x_1 - m x_2}{n - m}. A similar process for the y-coordinates, using vertical distances, yields y = \frac{n y_1 - m y_2}{n - m}. This proof emphasizes the opposite direction in external division compared to the internal case, where the ratio is positive without the sign flip. For example, consider points A(1, 2) and B(3, 4) divided externally in the 2:1 (m=2, n=1). The coordinates of P are \left( \frac{1 \cdot 1 - 2 \cdot 3}{1 - 2}, \frac{1 \cdot 2 - 2 \cdot 4}{1 - 2} \right) = \left( \frac{1 - 6}{-1}, \frac{2 - 8}{-1} \right) = (5, 6). This point lies outside the segment AB, extended beyond B. The formula requires m ≠ n to avoid , which would occur if the ratio led to an indeterminate ; otherwise, the point would coincide with one or be undefined. This external counterpart to internal division finds applications in concepts like harmonic divisions, where points divide segments in specific negative ratios.

Midpoint formula

The midpoint of a line segment in two-dimensional coordinate geometry is defined as the point that divides the segment in the ratio 1:1, meaning it lies exactly halfway between the endpoints. This formula arises as a special case of the internal division section formula when the ratio m:n = 1:1. For endpoints A(x_1, y_1) and B(x_2, y_2), the coordinates of the midpoint M are given by M = \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right). This expression simplifies the general section formula by averaging the coordinates directly, reflecting equal partitioning. A proof of the midpoint formula can be derived using vector averaging in coordinate geometry. Consider the position vectors of points A and B as \vec{A} = (x_1, y_1) and \vec{B} = (x_2, y_2). The midpoint M is the point whose position vector \vec{M} satisfies \vec{M} = \frac{\vec{A} + \vec{B}}{2}, which expands to the coordinate form above; this follows from the symmetry of the line segment and the property that the vector from A to M equals the vector from M to B in magnitude and direction opposition. Key properties of the midpoint include its role in the midpoint theorem for triangles, which states that the segment joining the midpoints of two sides of a triangle is parallel to the third side and half as long; this theorem implies the parallelogram law by demonstrating vector addition properties in geometric figures. For example, the midpoint of segment joining A(0,0) and B(4,6) is M(2,3), which geometrically represents the center of the segment and balances the coordinates equidistantly from both ends. The midpoint formula also aids in verifying segment properties via the distance formula, such as confirming that the distances from each endpoint to the are equal, establishing the .

Special cases and applications

Centroid of a triangle

The of a is defined as the point of concurrency of its three s, where each median connects a to the of the opposite side and is divided by the in the ratio 2:1, with the longer portion adjacent to the . This point represents the geometric center of the in terms of mass distribution when assuming equal masses at the . In two-dimensional , for a with vertices A(x_1, y_1), B(x_2, y_2), and C(x_3, y_3), the coordinates of the G are calculated as the of the vertices' coordinates: G = \left( \frac{x_1 + x_2 + x_3}{3}, \frac{y_1 + y_2 + y_3}{3} \right) This formula arises directly from the section formula applied to the s. To derive it, consider the from A to the M of side BC, where M = \left( \frac{x_2 + x_3}{2}, \frac{y_2 + y_3}{2} \right). Since G divides segment AM in the ratio 2:1 (two parts toward M, one part toward A), the section formula gives: G_x = \frac{2 \cdot \frac{x_2 + x_3}{2} + 1 \cdot x_1}{3} = \frac{x_1 + x_2 + x_3}{3} The y-coordinate follows analogously. Repeating this for the other medians yields the same result, confirming concurrency at this point. Key properties of the centroid include its role as the balance point for a uniform triangular lamina, where it serves as the center of mass under constant density. It always lies inside the triangle, regardless of the triangle's shape. In barycentric coordinates, the centroid corresponds to the weighted average with equal weights of \frac{1}{3} for each vertex, reflecting its position as the arithmetic mean. Additionally, the centroid provides the reference for computing moments of inertia in triangular regions; for instance, the second moment of area about a centroidal axis parallel to the base establishes the minimum inertia value before applying the parallel axis theorem. As an example, consider a with vertices at (0,0), (3,0), and (0,4). Applying the formula yields G = \left( \frac{0 + 3 + 0}{3}, \frac{0 + 0 + 4}{3} \right) = \left(1, \frac{4}{3}\right), which lies inside the triangle and divides each in the 2:1 .

Section formula in conic sections

The section formula plays a crucial role in parametrizing points that divide of conic sections, enabling the determination of coordinates for intermediate points on curves such as ellipses, parabolas, and hyperbolas. In coordinate geometry, this involves applying the internal or external formula to the parametric coordinates of chord endpoints, which lie on the conic. For a chord joining points (x_1, y_1) and (x_2, y_2) divided in the ratio m:n, the coordinates of the dividing point P are given by x = \frac{m x_2 + n x_1}{m + n}, \quad y = \frac{m y_2 + n y_1}{m + n} for internal division, allowing precise location of points along focal chords or other segments within the conic. This parametrization is particularly useful in parabolas, where can be analyzed using the standard form y^2 = 4ax. Consider the latus rectum, a focal to the axis with endpoints (a, 2a) and (a, -2a). The point dividing these endpoints in the 1:1 (the ) has coordinates (a, 0), which coincides with the of the parabola, illustrating how the section formula reveals key geometric properties. In ellipses, the section formula connects chord division to the e. Specifically, the vertices of the can be derived from points dividing the segment between a and the corresponding point on the directrix in the ratio e:1. For an with S at (ae, 0) and directrix point Z at (a/e, 0), the points A and A' dividing SZ in this ratio yield the vertices at (a, 0) and (-a, 0), with the center as the midpoint, directly tying the division ratio to e = c/a where c is the focal distance. Applications extend to calculating tangents and normals at these division points. For a point P dividing a chord in a conic, the tangent at P can be found by substituting its coordinates into the conic's tangent equation derived from parametric forms; similarly, the normal follows from the derivative or perpendicular condition. In parametric equations for conics—such as (at^2, 2at) for parabolas or (a \cos \theta, b \sin \theta) for ellipses—the section formula integrates seamlessly to locate such points, facilitating derivations in modern . While Apollonius of Perga's ancient treatise Conics (c. 200 BCE) established foundational properties of these curves through synthetic methods, the section formula's coordinate-based approach, developed post-Cartesians, has enabled analyses central to contemporary applications in and .

Three-dimensional geometry

Internal and external division

In three-dimensional coordinate , the section formula determines the coordinates of a point that divides the line segment joining two points A(x_1, y_1, z_1) and B(x_2, y_2, z_2) in a given m:n, either internally (where the point lies between A and B) or externally (where the point lies outside the segment). This generalization incorporates the z-coordinate while preserving the proportional division principle from lower dimensions. For internal division in the ratio m:n (meaning the distances satisfy AP:PB = m:n), the coordinates of the point P are given by P = \left( \frac{m x_2 + n x_1}{m + n}, \frac{m y_2 + n y_1}{m + n}, \frac{m z_2 + n z_1}{m + n} \right). This formula arises from the representation of the line through A and B. The equations of the line are \begin{align*} x &= x_1 + t (x_2 - x_1), \\ y &= y_1 + t (y_2 - y_1), \\ z &= z_1 + t (z_2 - z_1), \end{align*} where t is a scalar with t = 0 at A and t = 1 at B. For internal division, t = \frac{m}{m + n}, reflecting the fractional distance from A to P relative to AB. Substituting this value yields x = x_1 + \frac{m}{m + n} (x_2 - x_1) = \frac{(m + n) x_1 + m x_2 - m x_1}{m + n} = \frac{n x_1 + m x_2}{m + n}, with analogous results for y and z. An alternative derivation relies on distance proportions in 3D space. Since A, P, and B are collinear, the Euclidean distances satisfy AP = \frac{m}{m + n} AB and PB = \frac{n}{m + n} AB. Projecting the segment onto the coordinate planes (XY, XZ, YZ) preserves collinearity and ratios due to orthogonal projections, leading to the same proportional coordinates in each dimension, analogous to similar triangles in planar geometry. For external division in the ratio m:n (where P lies on the extension of AB such that AP:PB = m:n with PB in the opposite direction), the coordinates are P = \left( \frac{m x_2 - n x_1}{m - n}, \frac{m y_2 - n y_1}{m - n}, \frac{m z_2 - n z_1}{m - n} \right), assuming m \neq n. In the parametric form, t = \frac{m}{m - n}, which can yield t > 1 or t < 0 depending on the ratio, placing P outside the segment. Substituting gives the negative sign in the numerator, as the effective contribution from B opposes that from A. The distance proportion proof follows similarly, with signed distances ensuring AP/PB = m/n while accounting for the external position. As an illustration, consider points A(1, 2, 3) and B(4, 5, 6) divided internally in the ratio 1:2. Here, m = 1, n = 2, so P = \left( \frac{1 \cdot 4 + 2 \cdot 1}{3}, \frac{1 \cdot 5 + 2 \cdot 2}{3}, \frac{1 \cdot 6 + 2 \cdot 3}{3} \right) = (2, 3, 4). This vector interpolation approach aligns with the abstract position vector formulation, where \vec{P} = \frac{n \vec{A} + m \vec{B}}{m + n} for internal division.

Midpoint and centroid in 3D

In three-dimensional space, the midpoint of a line segment joining two points A(x_1, y_1, z_1) and B(x_2, y_2, z_2) is the point that divides the segment in the ratio 1:1, given by the coordinates \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}, \frac{z_1 + z_2}{2} \right). This formula extends the two-dimensional midpoint by incorporating the z-coordinate average, representing the geometric center of the segment where equal weights are balanced. For a tetrahedron with vertices A(x_1, y_1, z_1), B(x_2, y_2, z_2), C(x_3, y_3, z_3), and D(x_4, y_4, z_4), the centroid is the average of the vertex coordinates: \left( \frac{x_1 + x_2 + x_3 + x_4}{4}, \frac{y_1 + y_2 + y_3 + y_4}{4}, \frac{z_1 + z_2 + z_3 + z_4}{4} \right). This point can be derived using the section formula applied to the medians of the tetrahedron, where each median joins a vertex to the centroid of the opposite face. The centroid divides each median in the ratio 3:1, with the longer segment from the vertex to the centroid. To see this, consider the median from vertex A to the face centroid G = \frac{B + C + D}{3}; applying the section formula for division in 3:1 yields \frac{1 \cdot A + 3 \cdot G}{4} = \frac{A + B + C + D}{4}, confirming the average as the intersection point of all medians. The centroid serves as the center of mass for a tetrahedron of uniform density, ensuring equilibrium when supported at that point, and it acts as the balance point in vector terms where the resultant force vector sums to zero. As an example, consider a cube with side length 1 and vertices at (0,0,0) and (1,1,1); the midpoint of this space diagonal is (0.5, 0.5, 0.5), which lies at the cube's center. For a tetrahedron with vertices at (0,0,0), (1,0,0), (0,1,0), and (0,0,1), the centroid is at (0.25, 0.25, 0.25), equidistant in a balanced sense from the vertices. The centroid corresponds to the first moment of the mass distribution; higher moments, such as the second moment of inertia, measure distribution about this point and extend the concept to rotational dynamics.

Vector formulation

General section formula

The general section formula in vector notation offers a coordinate-free method to express the position vector of a point dividing the line segment joining two given points in a specified ratio. For internal division in the ratio m:n, where m and n are positive real numbers representing the proportional segments, the position vector \vec{P} of the dividing point P is \vec{P} = \frac{n \vec{A} + m \vec{B}}{m + n}, with \vec{A} and \vec{B} denoting the position vectors of points A and B, respectively. This weighted average ensures P lies between A and B, with the weights n/(m+n) for \vec{A} and m/(m+n) for \vec{B} summing to unity. For external division in the same ratio m:n (assuming n > m > 0 to place P outside the segment on the side of A), the formula adjusts to account for the opposing direction: \vec{P} = \frac{n \vec{A} - m \vec{B}}{n - m}. Here, the negative sign reflects the extension beyond the segment, yielding weights that still sum to 1 but with one negative. These expressions derive from affine combinations of vectors, where the point is a linear blend with coefficients summing to 1, equivalent to barycentric weights along the line. To derive the internal case, parameterize the line from A to B such that the displacement to P is the fraction m/(m+n) of \vec{B} - \vec{A}: \vec{P} = \vec{A} + \frac{m}{m+n} (\vec{B} - \vec{A}). Expanding using vector addition and scalar multiplication properties yields \vec{P} = \frac{n \vec{A} + m \vec{B}}{m + n}. The external derivation follows analogously, with the parameter exceeding 1 or becoming negative, leading to the subtraction term. The vector formulation's primary advantages include invariance under rotations and translations, as position vectors shift rigidly without altering relative ratios, and straightforward extension to arbitrary dimensions beyond 2D or 3D coordinate systems. As an illustration, consider \vec{A} = \langle 1, 2 \rangle and \vec{B} = \langle 3, 4 \rangle dividing internally in the ratio 1:1. Then \vec{P} = \frac{1 \cdot \langle 1, 2 \rangle + 1 \cdot \langle 3, 4 \rangle}{2} = \langle 2, 3 \rangle. This approach underpins applications like the centroid as an equal-weight vector average.

Relation to coordinate systems

The vector formulation of the section formula aligns directly with its coordinate-based counterpart in spaces, as position vectors are simply the coordinate representations of points relative to an . Specifically, when the components of the vector expression \vec{P} = \frac{n \vec{A} + m \vec{B}}{m + n} for internal division are , they the standard coordinate formula P(x, y) = \left( \frac{n x_A + m x_B}{m + n}, \frac{n y_A + m y_B}{m + n} \right) in two dimensions, demonstrating their equivalence in Cartesian systems. This correspondence extends to three dimensions, where position vectors \langle x, y, z \rangle map precisely to rectangular coordinates, allowing seamless translation between vector operations and coordinate calculations. In higher dimensions, the section formula generalizes through barycentric coordinates, expressed as \vec{P} = \frac{\sum_{i=1}^k w_i \vec{A_i}}{\sum_{i=1}^k w_i}, where w_i are non-negative weights summing to 1 for points within a in n-dimensional space; this affine combination enables the location of points dividing segments or simplices in arbitrary ratios across spaces of any . Unlike the rigid two- or three-dimensional coordinate formulas, this approach scales naturally to n > 3, supporting computations in abstract spaces without rederiving component-wise equations. This formulation finds practical use in through (), where a point on a is computed as \vec{P} = (1 - t) \vec{A} + t \vec{B} for $0 \leq t \leq 1, equivalent to internal division in ratio t : (1 - t), facilitating smooth transitions in rendering and . In physics, it underpins the center of mass calculation, \vec{R}_{cm} = \frac{\sum m_i \vec{r}_i}{\sum m_i}, a weighted that determines independently of the origin. More recently, in , weighted averages of embeddings—such as \vec{e} = \sum w_i \vec{v}_i / \sum w_i, where \vec{v}_i are word vectors—generate document representations, enhancing tasks in models like . Vector formulations offer advantages over pure coordinate systems in non-Euclidean spaces, as they operate on vectors at points, accommodating curved geometries like manifolds without assuming flat metric properties inherent to Cartesian grids. For instance, in or spherical spaces, vector-based weighted combinations preserve local linearity while coordinates would require metric-specific adjustments. To illustrate the equivalence in two dimensions, consider points A(1, 2) and B(5, 8) divided internally in the 2:3; the coordinate formula gives P\left( \frac{2 \cdot 1 + 3 \cdot 5}{5}, \frac{2 \cdot 2 + 3 \cdot 8}{5} \right) = P(3.4, 5.6), matching the vector output \vec{P} = \frac{2 \vec{A} + 3 \vec{B}}{5} = \langle 3.4, 5.6 \rangle.

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