Find the volume V obtained by rotating
f(x) = x (5-x) and g(x)= 8 - x(5-x) about
the y -axis.

I understand the height, which f(x)-g(x) since f(x) g(x), but I don't understand how they got radius, r = x. Can someone explain that to me. Thank you!

Question

Find the volume V obtained by rotating 
f(x) = x (5-x) and g(x)= 8 - x(5-x) about 
the y -axis.

I understand the height, which f(x)-g(x) since f(x) > g(x),  but I don't understand how they got radius, r = x. Can someone explain that to me. Thank you!

anonymous · Answer

your radius will be the values of the space between the two curves evaluated via integral from their intersection points, integrating with respect to y