This is a formula for calculating the 2th power of a number, for example if you want to calculate the 2th power of 5:
$\begin{align*} a&= 2\\ b&= 3 \end{align*}$
$ 5^2 = a^2 + b^2 + 2(ab)$
Sorry if that's not the right notation, but you'll get the point. Question is, what's the formula/method called?