04 4 a.m數 (26分到)

Sorry, I do not do MSN, but I can provide you with the answer.

You only have to integrate a small element of size dxdy from x=0 to sqrt(R^2-y^2) and for y=0 to 2. R=3 for x^2+y^2=9 (a circle of radius 3)
The element will make a volume of 2pi x (dxdy) when revolved around the y-axis.
So the total volume is

integral(y=0 to 2 integral(x=0 to sqrt(R^2-y^2) (2 pi x dx dy) ) )
= integral(y=0 to 2 [2 pi x^2 / 2] x=0 to sqrt(R^2-y^2) )
= integral(y=0 to 2 (pi (R^2-y^2) )
=[pi( R^2y - y^3/3) ] y=0 to 2
=pi (2R^2 - 2^3/3)
=pi (2 3^2 - 2^3/3)
=pi (18 - 8/3)
=46pi/3

呢條數做乜會26分咁多？！
6分都唔值喎