# Thread: Slope of polar curve

1. ## Slope of polar curve

Find the slope of the polar curve at the indicated point.
r=8(1+cos theta) theta=pi/4

I must be doing it wrong because I took the derivative of r and got:
dr/dtheta = -8sin theta= -4 radical 2.

Whats my error?

2. Originally Posted by krzyrice
Find the slope of the polar curve at the indicated point.
r=8(1+cos theta) theta=pi/4

I must be doing it wrong because I took the derivative of r and got:
dr/dtheta = -8sin theta= -4 radical 2.

Whats my error?
$\displaystyle r = 8(1 + \cos{\theta})$

$\displaystyle \frac{dr}{d\theta} = -8\sin{\theta}$.

So when $\displaystyle \theta = \frac{\pi}{4}$

$\displaystyle \frac{dr}{d\theta} = -8\sin{\frac{\pi}{4}}$

$\displaystyle = -8\left(\frac{\sqrt{2}}{2}\right)$

$\displaystyle = -4\sqrt{2}$.

3. yes that is what i got but the answer is 1-radical 2

4. Originally Posted by krzyrice
yes that is what i got but the answer is 1-radical 2
Then either the answer is wrong or you copied down the question wrongly.

5. weird. i didn't copy the problem wrong. it must be a coincidence because it seems the problem after that is wrong too