The question is:
$$\begin{aligned}\\ \text{Evaluate } \iint_R|x+y|\,dx\,dy \text{ over } R&:{|x|\leq 1, |y|\leq 1}\\ \end{aligned}$$
In my book it is given like:
$$\begin{align}\\ &\iint_{\Delta ABC}(x+y)dxdy+\iint_{\Delta ADB}(x+y)dxdy\\ &{=\begin{aligned}\\ \int_{x=-1}^1\int_{y=-x}^1(x+y)dxdy+\int_{x=-1}^1\int_{y=-1}^{-x}&-(x+y)dxdy\\ \end{aligned}\\}\\ \end{align}\\ \text{And so on..} $$ $$\text{I can't understand this solution. Can anyone please explain this?}$$
How to deal with absolute value functions in double integrals?