This is LaTeX code:

[latex]
<x,y>=\frac{1}{4}||x+y||^2-\frac{1}{4}||x-y||^2=\frac{1}{4}||A(x+y)||^2-\frac{1}{4}||A(x-y)||^2=\frac{1}{4}||Ax+Ay||^2-\frac{1}{4}||Ax-Ay||^2=<Ax,Ay>
[/latex]