Pythagorean theorem, also known as quotient height theorem and Pythagorean theorem, is a basic and important theorem in plane geometry.

Pythagorean theorem shows that the sum of squares of the lengths of two right-angled sides of a right-angled triangle on a plane (called hook length and head in ancient times) is equal to the square of the length of hypotenuse (called chord length in ancient times). On the other hand, if the sum of squares of two sides of a triangle on the plane is equal to the square of the third side, it is a right triangle (the opposite side of the right angle is the third side).

Pythagorean theorem is an elementary geometric theorem, one of the important mathematical theorems discovered and proved by human beings in the early days, one of the most important tools to solve geometric problems with algebraic ideas, and one of the ties between number and shape. "Hook three strands, four chords and five" is the most basic formula of Pythagorean theorem. On the other hand, if the sum of squares of two sides of a triangle on the plane is equal to the square of the third side, it is a right triangle (the opposite side of the right angle is the third side). Pythagorean array equation a? +b？ =c？ Positive integer group (a, b, c) of. That is to say, if the two right angles of a right triangle are A and B and the hypotenuse is C, then A? +b？ =c？ . It also has a long history in the history of Chinese mathematics, and it is the most important thing in the middle calculation. There is a description of "hooking three strands, four chords and five" in the Parallel Computation Classic of Zhou Dynasty, and Zhao Shuang's Notes on Parallel Computation describes the Pythagorean Theorem as "Pythagorean Theorem is a chord reality that multiplies each other. In addition to prescriptions, it is a string. " . Pythagoras discovered Pythagoras theorem in ancient Greece, so Pythagoras theorem is also called Pythagorean theorem.

A popular saying is to hook three strands, four strings and five. It means 3 2+4 2 = 5 2.