For 12, 13 and 14, draw the area and find
(a) the signed area bounded by $y = f(x)$ and the x-axis from
x = a to x = b.
(b) the absolute area bounded by $y = f(x)$ and the x-axis from
x = a to x = b.
12. $y = -\cos(x)$, $a = 0$, $b = 3\pi/2$.
13 $y = -x^2 + 2x + 3$, $a = 1$, $b = 4$.
14. $y = e^x - 1$, $a = -1$, $b = 1$.