The Pythagorean Theorem says that in any right triangle, the lengths of the hypotenuse and the other two sides are related by a simple formula.

So, if you know the lengths of any two sides of a right triangle,
you can use the Pythagorean Theorem to find the length of the third side:

The Pythagorean Theorem is true for all right triangles.
There are many ways, called proofs,
to show that this is true.