Vol of a cylinder...VCAA E2 2009

[you0006]

Hi guys, l've got two formulas for a cylinder

Vol= (pi) x (radius)^2 x height

and

Vol = 2 x pi x (radius)^2 + 2 x pi x radius x height


Now for this VCAA question, using r = 12 and h = 30
using the first eq I get 4320pi or 13571.1

by doing: pi x 12^2 x 30

Yet for with the second equation I get 1008pi or 3166.7
by doing: (2 x pi x 12^2) + (2 x pi x 12 x 30)


SO which one is correct?

Looking at the solutions VCAA says to use the first eq, so how come BOTH formulas don't work?

[DannyN]

Break it into two shapes. 1. half of a sphere 2. a cylinder
1. work out the volume of the half sphere, v=.5[4/3pi(12^3)
2.volume of cylinder                -->                v=30(12^2)pi

then add them together

[you0006]

I know how to work them out, i'm just puzzled as to why both formulas for the volume of a cylinder don't give the same answer

[xZero]

Hi guys, l've got two formulas for a cylinder

Vol= (pi) x (radius)^2 x height

and

Vol = 2 x pi x (radius)^2 + 2 x pi x radius x height


Now for this VCAA question, using r = 12 and h = 30
using the first eq I get 4320pi or 13571.1

by doing: pi x 12^2 x 30

Yet for with the second equation I get 1008pi or 3166.7
by doing: (2 x pi x 12^2) + (2 x pi x 12 x 30)


SO which one is correct?

Looking at the solutions VCAA says to use the first eq, so how come BOTH formulas don't work?

thats because 1st equation is for volume of cylinder, 2nd equation is for surface area of cylinder

[you0006]

Ahhh.. finally I will be able to sleep well at night!