Portal:Mathematics/Featured article/2007 25

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The Pythagorean theorem or Pythagoras' theorem is a relation in Euclidean geometry among the three sides of a right triangle. The theorem is named after the Greek mathematician Pythagoras, who by tradition is credited with its discovery, although knowledge of the theorem almost certainly pre-dates him (in China, for example). The theorem is as follows: "In any right triangle, the area of the square whose side is the hypotenuse (the side of a right triangle opposite the right angle) is equal to the sum of areas of the squares whose sides are the two legs (i.e. the two sides other than the hypotenuse)." If we let c be the length of the hypotenuse and a and b be the lengths of the other two sides, the theorem can be expressed as the equation
 * $$a^2 + b^2 = c^2\, $$

or, solved for c:
 * $$\sqrt{a^2 + b^2} = c. \,$$

This equation provides a simple relation among the three sides of a right triangle so that if the lengths of any two sides are known, the length of the third side can be found. A generalization of this theorem is the law of cosines, which allows the computation of the length of the third side of any triangle, given the lengths of two sides and the size of the angles between them. If the angle between the sides is a right angle it reduces to the Pythagorean theorem.