How to Solve It
How to Solve It: A New Aspect of Mathematical Method is a 1945 book by Hungarian-American mathematician George Pólya that provides a heuristic framework for solving mathematical problems through a structured four-step process: understanding the problem, devising a plan, carrying out the plan, and looking back.[1][2] The work emphasizes fostering curiosity and inventive thinking, applicable not only to mathematics but to any reasoned problem, such as proofs, puzzles, or practical challenges like bridge-building.[1] George Pólya (1887–1985) was a prominent mathematician known for contributions to complex analysis, probability, and combinatorics.[3] Born György Pólya in Budapest, Hungary, he earned his doctorate from the University of Budapest in 1912 and studied further at the University of Paris.[3] He served as a professor at ETH Zürich from 1914 to 1940, emigrating to the United States in 1940 amid World War II, and joined Stanford University in 1942, where he taught until his retirement in 1953 and continued as an emeritus professor until 1978.[3] The book draws from Pólya's lectures and experience as an educator, targeting both mathematics students and teachers to promote active learning and discovery over rote methods.[1] It features practical examples, including geometry and number theory problems, and a "dictionary of heuristics" listing strategies such as drawing auxiliary figures, using analogy, or working backwards from the desired solution.[1] Pólya stresses the importance of teacher guidance in encouraging students to explore problems heuristically, highlighting questions like "What is the unknown?" and "What is the condition?" to clarify objectives.[4] Since its initial publication by Princeton University Press, How to Solve It has established itself as a cornerstone of mathematical pedagogy, with a paperback reissue in 2014 and ongoing availability in print and digital formats.[1] As a bestselling title, it has influenced problem-solving instruction across disciplines, aiding millions in developing systematic approaches to challenges and underscoring the universal nature of mathematical thinking.[1]Background and Publication
Author and Context
George Pólya (1887–1985) was a Hungarian-American mathematician renowned for his contributions to complex analysis, probability theory, and combinatorics. Born György Pólya on December 13, 1887, in Budapest, Hungary, he initially studied law at the University of Budapest before switching to mathematics, earning his doctorate in 1912. Pólya held a position as Privatdozent at ETH Zurich starting in 1914, becoming an extraordinary professor there in 1920 and an ordinary professor in 1928, and remained until 1940. In 1940, amid the escalating tensions of World War II, he emigrated to the United States, initially teaching at Brown University (1940–1941) and Smith College (1941–1942) before joining the faculty at Stanford University in 1942, where he taught until his retirement in 1953 and continued as professor emeritus thereafter.[3] Pólya's intellectual influences were deeply rooted in his extensive teaching experiences across Europe and the United States, where he grew frustrated with students' struggles to tackle non-routine problems despite proficiency in routine calculations. At ETH Zurich, under the influence of mentors like Adolf Hurwitz and Fejér, he began emphasizing inductive and heuristic approaches to mathematics, viewing teaching as an active process that fosters discovery rather than passive absorption of definitions and proofs. This perspective intensified during his time in the U.S., particularly at Stanford, where he collaborated with colleagues like Gábor Szegő and observed the limitations of traditional pedagogy in preparing students for creative problem-solving. Pólya advocated for "active learning," arguing that mathematics education should simulate the exploratory nature of mathematical research to bridge the gap between routine exercises and genuine innovation.[3][5] The pre-publication context of How to Solve It emerged from Pólya's lectures and articles in the 1930s and 1940s, which sought to remedy deficiencies in mathematical pedagogy exacerbated by World War II's disruptions to educational systems and faculty migrations. Initially drafted in German before his 1940 departure from Europe, the work drew on his earlier problem-solving compilations and heuristic explorations, reflecting a broader effort to democratize mathematical invention amid wartime instability. Pólya encapsulated this motivation in his observation that mathematicians do not work by definition and deduction alone, underscoring the essential role of plausible reasoning and intuition in discovery—a theme that permeated his pedagogical innovations.[3][6]Publication Details
How to Solve It was first published in 1945 by Princeton University Press as a 253-page volume aimed primarily at teachers and students of mathematics, rather than professional mathematicians.[1][7] The book is structured into short chapters featuring annotations and questions to guide readers, along with an appendix serving as a teacher's manual, and it incorporates 46 worked examples spanning topics from arithmetic to calculus.[1][8] A second edition appeared in 1957, incorporating minor revisions by the author while preserving the original framework.[9] By 2025, the book had been translated into over 20 languages, reflecting its global reach, and recent editions include digital formats such as eBooks.[10][11] A common reprint bears the ISBN 978-0-691-09193-2.[12] The original edition achieved sales exceeding 1 million copies by the 2000s, underscoring its enduring popularity.[10] Following its release shortly after World War II, it rapidly established itself as a standard text in U.S. mathematics education.[1]Core Framework
Understanding the Problem
Understanding the problem constitutes the foundational step in George Pólya's four-step framework for mathematical problem-solving, which also encompasses devising a plan, carrying out the plan, and looking back. This initial phase emphasizes a thorough diagnostic analysis to ensure clarity regarding the problem's requirements and constraints, preventing misdirected efforts in subsequent stages. Pólya stresses that effective comprehension requires deliberate attention, as superficial reading often leads to errors in interpretation. To achieve this understanding, Pólya recommends posing a series of heuristic questions as a systematic checklist. These include: What is the unknown? What are the data? What is the condition? Is the condition sufficient to determine the unknown? Additional queries focus on verifying comprehension, such as whether all terms in the problem statement are clear and if the condition is possible to satisfy without being redundant, insufficient, or contradictory. By addressing these, one distinguishes the essential elements from irrelevancies, ensuring the problem is restated accurately in one's own terms.[13] Practical tactics further aid this process. Drawing a figure or diagram can visualize relationships, particularly in spatial problems; introducing suitable notation standardizes variables; and separating the condition's components into explicit parts clarifies dependencies. Pólya advises spending ample time here, as thorough understanding lays the groundwork for efficient planning.[13] For instance, consider a geometry problem requiring the foot of the perpendicular from a given point to a line defined by its equation. Sketching the line and marking the point reveals the givens (line equation and point coordinates) versus the seek (intersection point), highlighting any implicit assumptions about the plane and prompting checks on whether the data suffice for computation. This visualization transforms an abstract statement into a concrete setup, facilitating identification of key relations.[14]Devising a Plan
In George Pólya's framework for mathematical problem-solving, the phase of devising a plan follows the initial understanding of the problem and focuses on generating viable strategies to reach a solution. This step requires creative exploration of potential methods, often through pattern recognition and the adaptation of familiar techniques, to outline a path forward. Pólya defines having a plan as knowing, at least in outline, the essential steps needed to solve the problem, emphasizing that this process relies on heuristic suggestions rather than rigid rules.[1] A primary approach in this phase is the use of analogy, which involves comparing the target problem to previously solved or familiar ones to transfer applicable strategies. Pólya advises questions such as "Do you know a related problem?" or "Can you think of it in terms of another, more accessible problem?" to identify similarities, enabling the solver to adapt known solutions. For instance, when tackling a complex puzzle, one might draw an analogy to a similar chess endgame, recognizing shared structural patterns and breaking the puzzle into manageable subgoals like isolating key pieces or testing sequential moves. Another key heuristic is working backwards, where the solver begins from the desired goal and traces reversible steps toward the given conditions, particularly effective when forward progress is obscured.[1] Pólya further recommends introducing auxiliary elements to simplify or reframe the problem, such as adding special cases, parameters, or related subproblems that bypass direct obstacles. He illustrates this by noting that "human superiority consists in going around an obstacle, in going around it by some suitable auxiliary problem," highlighting how such additions can reveal hidden connections. Complementary heuristics include induction, which generalizes patterns observed from specific instances to broader principles, and specialization, starting with simpler or particular cases to build intuition before addressing the general form. These methods encourage iterative refinement, as plans remain tentative and subject to adjustment based on emerging insights during exploration.[1] Central to devising a plan is what Pólya terms "plausible reasoning," a provisional mode of thinking that relies on partial evidence and intuitive guesses to guide discovery, rather than demanding immediate proof. He warns against premature rigor, explaining that "heuristic reasoning is reasoning not regulated and verified by strict logic... [it is] provisional and plausible only, whose purpose is to discover the solution." This approach underscores the exploratory nature of planning, where initial strategies serve as educated conjectures that evolve, ensuring flexibility in the face of uncertainty.[1]Carrying Out the Plan
The third step in George Pólya's problem-solving framework involves executing the devised plan with precision and attentiveness, transforming the conceptual strategy into a concrete solution. This phase requires following the blueprint established earlier while emphasizing rigorous verification to ensure each action aligns with the problem's requirements. Pólya describes it as a process where the solver must carry out the plan methodically, underscoring that successful implementation depends on disciplined effort rather than mere inspiration.[14] Central to this principle are tactics such as keeping the overall plan in view for guidance, yet remaining flexible to minor modifications if obstacles emerge during execution, thereby allowing for adaptive responses without abandoning the core approach. Solvers should check each step rigorously, confirming its correctness through clear reasoning or proof, and verify both arithmetic computations and logical inferences at every stage to prevent propagation of errors. Maintaining consistent notation is equally essential, as uniform use of symbols and terms facilitates clarity and reduces the risk of misinterpretation amid detailed work. These practices highlight the emphasis on careful computation, where routine verifications guard against common oversights in numerical or deductive processes.[14] A representative example of this phase occurs in executing a mathematical proof, such as demonstrating that the sum of the first n odd numbers equals n^2. Following the plan, the solver fills in algebraic steps—perhaps inducting from the base case and assuming the statement for k before proving for k+1—while cross-checking each derivation against initial assumptions and prior steps to ensure logical coherence. This illustrates the balance between routine mechanical labor, like performing standard inductions, and creative adaptation if an unexpected gap requires slight plan tweaks, a duality Pólya positions as vital yet often undervalued in favor of inventive planning. The principle thus addresses the foundational mechanical elements of problem-solving, where diligence in execution sustains the creativity of earlier phases.[14]Looking Back
The "Looking Back" phase in George Pólya's problem-solving framework represents the reflective culmination of the process, where the solver evaluates the obtained solution to verify its validity and uncover deeper insights. This step encourages a critical examination beyond mere completion, transforming the resolution of a single problem into an opportunity for broader understanding and skill development. Pólya emphasizes that this reflection not only confirms correctness but also reveals connections and extensions that enhance future problem-solving capabilities.[1] Central to this phase are a set of guiding review questions designed to probe the solution's robustness and applicability. These include: Can you check the result? This prompts verification of the outcome's accuracy, often through direct substitution or computational tests. Can you derive it differently? Here, the solver seeks alternative proofs or methods to confirm the argument's soundness and potentially simplify the approach. Can you use it elsewhere? This question explores whether the result or technique applies to related problems, identifying reusable patterns. Finally, can you generalize it? This involves extending the solution to a wider class of scenarios, such as varying parameters or relaxing assumptions. These questions, drawn directly from Pólya's methodology, foster a systematic review that turns verification into discovery.[15][16] Practical tactics in the "Looking Back" phase include verifying the solution through alternative methods, such as reversing steps or using independent computations, to build confidence in its reliability. Solvers are also urged to consider the solution's broader implications, like identifying emergent patterns that signal strategies for similar challenges, and to assess how the method might adapt to variations in the problem's conditions. This reflective practice not only catches errors but also enriches conceptual grasp by linking the specific case to general principles.[17] A concrete illustration of these tactics appears in solving a quadratic equation, such as x^2 - 5x + 6 = 0. After carrying out the plan via factoring to find roots x = 2 and x = 3, one checks the result by substitution: plugging x = 2 yields $4 - 10 + 6 = 0, confirming validity. To derive differently, the quadratic formula x = \frac{5 \pm \sqrt{25 - 24}}{2} yields the same roots, reinforcing the argument. Looking further, exploring related inequalities like x^2 - 5x + 6 > 0 reveals the solution's utility in interval analysis, generalizing to conic sections or optimization problems. This process exemplifies how verification evolves into extension.[1] The iterative nature of "Looking Back" distinguishes it as a dynamic step, where insights from one solution iteratively refine methods for a class of problems, promoting efficiency in subsequent encounters. This phase cultivates mathematical maturity by encouraging habitual reflection that deepens intuition and adaptability over time.[16][1]Problem-Solving Heuristics
Key Heuristics
In George Pólya's "How to Solve It," heuristics are described as practical rules of thumb for plausible reasoning, serving as flexible guides to stimulate creative thinking rather than rigid algorithms that ensure a solution. While the heuristics support the book's four principles—particularly in devising a plan—they encourage solvers to explore connections, simplify complexities, and iterate thoughtfully. Pólya enumerates around 15 key suggestive questions as heuristics in the "Devising a Plan" section, each prompting a specific strategy with a rationale rooted in leveraging existing knowledge or restructuring the problem. Do you know a related problem? This heuristic urges recalling a previously solved issue with structural similarities, as prior successes often provide transferable insights into the current challenge.[8] Look at the unknown and try to think of a familiar problem having the same or similar unknown. By focusing on the goal, this approach identifies analogous situations where the target variable or outcome matches, facilitating adaptation of known methods.[8] Here is a problem related to yours and solved before. Could you use it? This prompts direct application of a solved case, rationalized by the efficiency of building on established results to avoid reinventing approaches.[8] Could you use its result? Exploiting the conclusion of a related problem simplifies computation, as outcomes can often serve as shortcuts or lemmas in the new context.[8] Could you use its method? This encourages adopting the technique from a similar problem, justified by the pattern that effective strategies recur across comparable scenarios.[8] Should you introduce something auxiliary to make its use possible? Adding temporary elements, such as extra variables or constructions, bridges gaps between known solutions and the present one, easing integration.[8] Could you restate the problem? Rephrasing clarifies ambiguities or reveals overlooked angles, rationalized as a way to refresh perspective without altering the core conditions.[8] Could you imagine a more accessible related problem? Considering a simpler variant builds intuition incrementally, as easier cases often illuminate paths to the original's resolution.[8] A more general problem? Tackling a broader formulation can paradoxically simplify the specific case, per the inventor's paradox, by exposing unifying principles.[8] A more special problem? Narrowing to a particular instance reduces complexity, allowing verification of conjectures that extend to the full problem.[8] An analogous problem? Drawing parallels to dissimilar domains leverages transferred knowledge, as structural resemblances transcend surface differences.[8] Could you solve a part of the problem? Partial solutions decompose the whole, rationalized by the manageability of subsets that recombine into a complete answer.[8] Could you derive something useful from the data? Extracting implications from givens advances forward reasoning, as hidden relations in the inputs often point toward the unknown.[8] Have you used all the data? This checks for overlooked information, ensuring comprehensive utilization to avoid incomplete analyses.[8] Have you used the whole condition? Verifying full engagement with constraints prevents errors from partial interpretations, promoting thoroughness.[8]Illustrative Examples
Pólya demonstrates the application of heuristics through concrete problems drawn from arithmetic, geometry, and algebra, showing how they facilitate both routine calculations and more inventive insights. In arithmetic examples, such as measuring a certain quantity with containers of given sizes, the working backwards heuristic proves particularly effective. For instance, to measure exactly 6 quarts using only a 4-quart pail and a 9-quart pail, the solver works backwards from the desired amount, considering sequences of filling, emptying, and pouring that lead to the goal, such as starting from states where one pail has 6 quarts.[8] Geometry examples highlight the use of symmetry to simplify divisions of figures. In one case involving triangles, Pólya applies symmetry to divide an equilateral triangle into smaller congruent parts, such as four smaller equilateral triangles by connecting midpoints, exploiting the figure's rotational and reflectional properties to count or construct without exhaustive enumeration.[8] Algebraic illustrations often involve Diophantine equations solved via auxiliary cases. Pólya introduces simpler auxiliary problems to tackle equations seeking integer solutions, such as breaking down a complex linear Diophantine equation into special cases (e.g., modulo reductions) that illuminate the general form, allowing the solver to build up to the complete set of solutions.[8] These examples are presented in Pólya's "short accounts" format—concise narratives that trace the problem-solving journey, blending routine steps like verification with inventive leaps prompted by heuristics, thereby bridging mechanical computation and creative discovery. Annotations accompanying the accounts offer guidance for teachers on adapting them for instruction, emphasizing questioning over direct answers.[8] To illustrate the integration of heuristics across the four principles, consider the problem of finding two positive numbers whose sum is 78 and product is 1296.- Understanding the problem: Identify the unknowns as two positive numbers x and y such that x + y = 78 and xy = 1296.
- Devising a plan: Apply the heuristic of restating the problem; recognize it leads to the quadratic equation t^2 - 78t + 1296 = 0, or consider related problems like solving quadratics.
- Carrying out the plan: Solve the quadratic: discriminant $78^2 - 4 \cdot 1296 = 6084 - 5184 = 900 = 30^2, so t = \frac{78 \pm 30}{2}, yielding t = 54 or t = 24. Thus, the numbers are 24 and 54.
- Looking back: Verify $24 + 54 = 78 and $24 \cdot 54 = 1296; reflect on using the sum and product to form the quadratic as a general method for such problems.