Pythagoras Theorem states that -

### "In a right angle triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides".

Let us understand same using figure as follows:

Pythagoras Theorem

Here, in △ABC, m∠B=90°,

∴ the side opposite to ∠B will the hypotenuse (longest side).

The longest side of a right angle triangle is known as hypotenuse.

So, as per the theorem stated above,

(Hypotenuse)2∴AC2=(side 1)2+(side 2)2=AB2+BC2
Play with the Pythagorean triplets here.