Pythagorean Theorem: If the lengths of two right-angled sides of a right-angled triangle are A and B respectively and the length of the hypotenuse is C, then A 2+B 2 = C 2.

Inverse Theorem of Pythagorean Theorem: If the lengths of three sides of triangle A, B and C satisfy A 2+B 2 = C 2, then the triangle is a right triangle.

A proposition that is proved to be correct is called a theorem.

We call two propositions with opposite topics and conclusions reciprocal propositions. If one of them is called the original proposition, then the other is called its inverse proposition. (Example: Pythagorean Theorem and Pythagorean Theorem Inverse Theorem)