What is Pythagoras' Theorem and how do I use it?

Pythagoras' Theorem is used to find the length of the hypotenuse (longest side, opposite the 90 degree angle) of a right angled triangle when the length of the other 2 sides are known.The equation is a²+b²=c²For example, if a triangle has sides of 4 cm and 3 cm.Say a = 4cm and b = 3cm. Then, c² =4²+3³ = 16 + 9 = 25and c = 5
4cm |\ |_\ 3cm