# Radius of cylinder

• Sep 18th 2012, 03:41 AM
Mimmi
The radius of a cylinder is quadrupled, determine the percent the volume increase.
Let the height twice as large as the radius.
Assume that the cylinder grows without changing shape.
V=pi*r^2 *h

So this is my task. Normally we get a % the radius increase, and no info about the height, so im kinda stuck.
Dont know if i can use the same method, and i dont know how many % quadrupled would be xD haha, i prolly should.
• Sep 18th 2012, 03:50 AM
MarkFL
Re: Radius of cylinder
If a 3D solid has all of its linear measures increase by a factor \$\displaystyle k\$, then the volume increases by the factor \$\displaystyle k^3\$.
• Sep 18th 2012, 05:14 AM
HallsofIvy
Re: Radius of cylinder
Quote:

Originally Posted by Mimmi
The radius of a cylinder is quadrupled, determine the percent the volume increase.

If you quadruple the radius only this quadruples the volume

Quote:

Let the height twice as large as the radius.
Now this becomes a different question!

Quote:

Assume that the cylinder grows without changing shape.
V=pi*r^2 *h
h= 2r so V= pi* r^2*(2r)= 2pi*r^3

Replacing r with 4r (quadupeling r) gives 2pi*(4r)^3= 4^3(2pi*r^3), which is 4^3= 64 times as large as the value before quadrupling.

[quorte]So this is my task. Normally we get a % the radius increase, and no info about the height[/quote]
Yes, you do. You are told the height is twice the radius.

Quote:

, so im kinda stuck.
Dont know if i can use the same method, and i dont know how many % quadrupled would be xD haha, i prolly should.
• Sep 19th 2012, 02:39 AM
Mimmi
Re: Radius of cylinder
ty!