# Maths help!

1. Feb 5, 2005

### footprints

How do you change this $$A = 720 \pi - \frac{4}{3} \pi r^3 + 3 \pi r^2$$ into $$A = 5 \pi (\frac{144}{r} + \frac{r^2}{3})$$?

2. Feb 5, 2005

### Galileo

Uuhm. You don't?

3. Feb 5, 2005

### The Bob

$$A = 720 \pi - \frac{4}{3} \pi r^3 + 3 \pi r^2$$

$$A = \pi (720 - \frac{4}{3} r^3 + 3 r^2)$$

$$A = \pi (\frac{2160}{3} - \frac{4}{3} r^3 + \frac{9}{3} r^2)$$

$$A = \pi (\frac{2160 - 4 r^3 + 9 r^2}{3})$$

$$A = \frac{1}{3}\pi (2160 - 4 r^3 + 9 r^2)$$

After this I have no idea at the present moment.

4. Feb 5, 2005

### dextercioby

The two expressions are not equal,which means you cannot find a chain of correct equalities which would bring one into another.

Daniel.

5. Feb 5, 2005

### christinono

Are you sure you typed them in right? They don't seem equivalent to me either.

6. Feb 5, 2005

### The Bob

Oh good. I thought it was just me being stupid.

7. Feb 5, 2005

### footprints

You guys are right! Their not equal. After redoing it two times, I got the right equation. :grumpy: Here it is: $$A = \frac{720 \pi - \frac{4}{3} \pi r^3}{r} + 3 \pi r^2$$. Thanks for the help guys!

8. Feb 5, 2005

### christinono

So do you still need help, or you know how to change one into the other?

9. Feb 6, 2005

### footprints

No I don't need help.