Pythagoras Theorem states that -
Let us understand same using figure as follows:
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.