Pythagoras’ theorem states that in a right angled triangle, on making a square on each of the three sides, we find that the biggest square has the exact same area as the area of the other two squares put together. The Pythagorean Theorem is named after the Greek mathematician Pythagoras, who by tradition is credited with its discovery and proof.
In other words consider the right angled triangle shown below having three sides a, b and c, then theorem is as follows:
The longest side of the triangle is called the "hypotenuse", so the formal definition of the theorem can be stated as, “In a right angled triangle the square of the hypotenuse is equal to the sum of the squares of the other two sides".
If we know the lengths of any two sides of a right angled triangle, then Pythagoras' Theorem allows us to find the length of the third side.