Ok, quite a simple Question.

The base of a solid S is the region in the xy plane enclosed by the parabola y^2 = 4g and the line x=4, and each cross section perpendicular to the x axis is a semi-ellipse with the minor axis one-half of the major axis.

Show that the area of the semi ellipse at x=h is pi.h

(you may assume A = pi.ab)

Ok, here's my question. Why is a = y and b = y/2. I dont get this