We have looked at two examples where the theorem is verified.
In mathematics, just verifying a result for two cases does not make it a theorem. A theorem is a general statement that must be proved for ALL cases, irrespective of the numbers we choose in the example.
So if we can represent the sides of the triangle by variables say a and b for the two smaller sides and c for the hypotenuse and then show the relation
a squared plus b squared equals c squared
then we have shown that the result is true for all values of a, b, and c which then proves the theorem.