词条 | Pythagorean theorem |
释义 | Pythagorean theorem mathematics 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. ![]() ![]() 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.) ![]() ![]() 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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