# Pythagoras's Theorem/Proof 5

## Theorem

Let $\triangle ABC$ be a right triangle with $c$ as the hypotenuse.

Then:

$a^2 + b^2 = c^2$

## Proof

The two squares both have the same area, that is, $\paren {a + b}^2$.

The one on the left has four triangles of area $\dfrac {a b} 2$ and a square of area $c^2$.

The one on the right has four triangles of area $\dfrac {a b} 2$ and two squares: one of area $a^2$ and one of area $b^2$.

Take away the triangles from both of the big squares and you are left with $c^2 = a^2 + b^2$.

$\blacksquare$

## Source of Name

This entry was named for Pythagoras of Samos.

## Historical Note

This proof is the basis of the Aldous Huxley short story Young Archimedes.