When finding the area enclosed by two curves,and are areas under the x axis. why don't I separate the areas above and below the x-axis for each curve, add them together, then take away the area under the curve for the other graph?
I'm asking because my textbook says you can just put everything into one expression,
$\int _a^bf\left(x\right)-g\left(x\right)dx\:$
but this doesn't make sense to me - I just can't visualise why.