This is a geometrical proof of Pythagorean theorem. We begin with our triangle:

Now we use the hypotenuse as one side of a square:

Now for the proof. We have a large square, with each side of length $a+b$ , which is subdivided into one smaller square and four triangles. The area of the large square must be equal to the combined area of the shapes it is made out of, so we have \begin{eqnarray} \left(a+b\right)^2 & = & c^2 + 4\left(\frac{1}{2}ab\right)\nonumber\\ a^2 + b^2 + 2ab & = & c^2 + 2ab \nonumber\\ a^2 + b^2 & = & c^2 \end{eqnarray}