# Math Help - Circle Problemo!

1. ## Circle Problemo!

I have a study guide for a test that I have in a few hours.. and I don't understand these problems! Help would would be great!
A=1/2*Pi*r^2*theta

dA/dthetha=?

da/dr=?

dtheta/dr=?

much thanks for any help given.

2. We have $A = 0.5 \pi r^2 \theta$

When asked to find $\frac{dA}{d\theta}$ for example, all you need to do is differentiate with respect to $\theta$.

Following that, we have:

$\frac{dA}{d \theta } = \frac{d}{d \theta } [0.5 \cdot\pi r^2 \cdot \theta] = 0.5 \pi r^2$

And you find $\frac{dA}{dr}$ by doing the same thing. To find $\frac{d \theta}{dr}$ though, you'll have to express $\theta$ as a function of r first..

3. Ok so would that mean

da/dr=d/dr(.5*Pi*r^2*thetha) = .5Pi*2R*theta = Pi*R*Theta?

with that said I thnk my answer is right..

4. No, differentiating with respect to r does NOT mean "dividing by r"! Have you ever taken Calculus? If not why are you doing these problems?

5. hey man I just had a brain cramp, your right. I R^2 =2R I just forgot to edit the /r out.
And im taking calculus now I have an A right now like Chain rule, etc.. is all easy to me I just never understood implicit diff.

thnnks for the help guyz.