Here is a 2D region R with bottom boundary curve low[x] = x(x - 1) and with top boundary curve high[x] = 2x (1 - x): X ab V W s = 2 x \\ ( 1 - x\\ ) R . 1 0.2 0 .4 0. 6 0. 8 . 1 y = x \\(x - 1\\) . 2 Say why it's natural to calculate Integrate[x + y, y, {x, R, ""}] by integrating with respect to y first. Next, calculate Integrate[x + y, y, {x, R, ""}]