Pythagorean theorem
The Pythagorean theorem applies to every right-angled triangle
Explanation
The picture shows 3 identical rectangular triangles, ∆ ABC, ∆ CBD and ∆ ACD.
The proportions of the sides are
and
so that
Because AD + BD = AB, the Pythagorean theorem follows directly from this
a2 + b2 = c2
Additional information
The old Greek mathematician Pythagoras (572 BC - 500 BC) is famous for his number theory.