# Pythagorean theorem

Pythagorean theorem
the theorem that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides.
[1905-10]

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Rule relating the lengths of the sides of a right triangle.

It says that the sum of the squares of the lengths of the legs is equal to the square of the length of the hypotenuse (the side opposite the right angle). That is, a2 + b2 = c2, where c is the length of the hypotenuse. Triads of whole numbers that satisfy it (e.g., 3, 4, and 5) are called Pythagorean triples. See also law of cosines; law of sines.

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proposition number 47 from Book I of Euclid's Elements, the well-known geometric theorem that the sum of the squares on the legs of a right triangle is equal to the square on the hypotenuse (the side opposite the right triangle)—or, in familiar algebraic notation, a2 + b2 = c2. Although the theorem has long been associated with the Greek mathematician-philosopher Pythagoras (c. 580–500 BC), it is actually far older. Four Babylonian tablets, circa 1900–1600 BC, indicate some knowledge of the theorem, or at least of special integers known as Pythagorean triples that satisfy it. Similarly, the Rhind papyrus, dating from about 1650 BC but known to be a copy of a 200-year-old document, indicates that the Egyptians knew about the theorem. Nevertheless, the first proof of the theorem is credited to Pythagoras.

According to the Syrian historian Iamblichus (c. AD 250–330), Pythagoras was introduced to mathematics by Thales of Miletus and his pupil Anaximander. In any case, it is known that Pythagoras traveled to Egypt about 535 BC to further his study, was captured during an invasion in 525 BC by Cambyses II of Persia and taken to Babylon, and may possibly have visited India before returning to the Mediterranean. Pythagoras soon settled in Croton (now Crotone, Italy) and set up a school, or in modern terms a monastery (see Pythagoreanism), where all members took strict vows of secrecy, and all new mathematical results for several centuries were attributed to his name. Thus, not only is the first proof of the theorem not known, there is also some doubt that Pythagoras himself actually proved the theorem that bears his name. Some scholars suggest that the first proof was the one shown in the figure—>. It was probably independently discovered in several different cultures.

Book I of the Elements ends with Euclid's famous “windmill” proof of the Pythagorean theorem. (See sidebar: Euclid's Windmill.) Later in Book VI of the Elements, Euclid delivers an even easier demonstration using the proposition that the areas of similar triangles are proportionate to the squares of their corresponding sides. Apparently, Euclid invented the windmill proof so that he could place the Pythagorean theorem as the capstone to Book I. He had not yet demonstrated (as he would in Book V) that line lengths can be manipulated in proportions as if they were commensurable numbers (integers or ratios of integers). The problem he faced is explained in the sidebar: Incommensurables.

A great many different proofs and extensions of the Pythagorean theorem have been invented. Taking extensions first, Euclid himself showed in a theorem praised in antiquity that any symmetrical regular figures drawn on the sides of a right triangle satisfy the Pythagorean relationship: the figure drawn on the hypotenuse has an area equal to the sum of the areas of the figures drawn on the legs. The semicircles that define Hippocrates of Chios's lunes are examples of such an extension. (See sidebar: Quadrature of the Lune.)

In the Nine Chapters on the Mathematical Procedures (or Nine Chapters), compiled in 1st century AD China, several problems are given, along with their solutions, that involve finding the length of one of the sides of a right triangle when given the other two sides. In the Commentary of Liu Hui (mathematics, East Asian), from the 3rd century, Liu Hui offered a proof of the Pythagorean theorem that called for cutting up the squares on the legs of the right triangle and rearranging them (“tangram style”) to correspond to the square on the hypotenuse. Although his original drawing does not survive, the next figure—> shows a possible reconstruction.

The Pythagorean theorem has fascinated people for nearly 4,000 years; there are now an estimated 367 different proofs, including ones by the Greek mathematician Pappus of Alexandria (flourished c. AD 320), the Arab mathematician-physician Thābit ibn Qurrah (Thābit ibn Qurra) (c. 836–901), the Italian artist-inventor Leonardo da Vinci (1452–1519), and even U.S. President James Garfield (Garfield, James A.) (1831–1881).

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Universalium. 2010.

### См. также в других словарях:

• Pythagorean theorem — n. Geom. the theorem that in a right triangle the hypotenuse squared is equal to the sum of the squares of the other sides (i.e., c2=a2+b2) …   English World dictionary

• Pythagorean theorem — See also: Pythagorean trigonometric identity The Pythagorean theorem: The sum of the areas of the two squares on the legs (a and b) equals the area of the square on the hypotenuse (c) …   Wikipedia

• Pythagorean theorem — noun a) A mathematical theorem which states that the square of the length of the hypotenuse of a right triangle is equal to the sum of the squares of those of the two other sides. b) A generalization of the Pythagorean theorem (1) to Hilbert… …   Wiktionary

• Pythagorean Theorem —    Pythagoras, the famous Greek philosopher and mathematician of the sixth century B.C., was born in Samos, off the coast of Asia Minor. As a mature man he immigrated to Croton, in southern Italy. Here he founded a brotherhood of religious… …   Dictionary of eponyms

• Pythagorean theorem — noun Date: 1743 a theorem in geometry: the square of the length of the hypotenuse of a right triangle equals the sum of the squares of the lengths of the other two sides …   New Collegiate Dictionary

• Pythagorean theorem — Pythag′ore′an the′orem n. math. the theorem that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides • Etymology: 1905–10 …   From formal English to slang

• pythagorean theorem — noun Usage: usually capitalized P : a theorem in geometry: the square of the length of the hypotenuse of a right triangle equals the sum of the squares of the lengths of the other two sides …   Useful english dictionary

• Pythagorean theorem — mathematical theory developed by Pythagoras (Greek mathematician) …   English contemporary dictionary

• Pythagorean expectation — is a formula invented by Bill James to estimate how many games a baseball team should have won based on the number of runs they scored and allowed. Comparing a team s actual and Pythagorean winning percentage can be used to evaluate how lucky… …   Wikipedia

• Pythagorean — means of or pertaining to the ancient Ionian mathematician, philosopher, and music theorist Pythagoras. See:Philosophy* Pythagoreanism an Egyptian influenced Neo Platonic philosophy named after the Greek philosopher and mathematician,… …   Wikipedia

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