# Thread: [SOLVED] Need help with related rates problem.

1. ## [SOLVED] Need help with related rates problem.

The Problem:
Grain is pouring out of a hole in the bottom of a large inverted cone with a vertical angle of 90 degrees at the rate of 7 m^3/min. Find the rate, in cm/s, at which the depth of grain in the cone is changing when the depth is 1.2m.

I don't understand how I find the radius.

2. Originally Posted by swatpup32
The Problem:
Grain is pouring out of a hole in the bottom of a large inverted cone with a vertical angle of 90 degrees at the rate of 7 m^3/min. Find the rate, in cm/s, at which the depth of grain in the cone is changing when the depth is 1.2m.

I don't understand how I find the radius.
were you given the dimensions of the cone?

3. No I was not.

4. I believe you were probably given that the vertex angle of the cone is 90 degrees ...

sketch a picture and you will see that r = h for any amount of grain in the cone.

$V = \frac{\pi}{3}h^3$

can you proceed from here?

5. Well I'm not a hundred percent sure what the vertex angle means?

6. Originally Posted by swatpup32
Well I'm not a hundred percent sure what the vertex angle means?
the vertex angle is the angle at the pointy end of the cone

7. Okay but I still don't see how that makes the height = radius.

Edit: never mind I figured out what you meant.
Thanks for the help.