# Slope of polar curve

#### 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?

#### Prove It

MHF Helper
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 $$\displaystyle \theta = \frac{\pi}{4}$$

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

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

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

#### krzyrice

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

#### Prove It

MHF Helper
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.

#### krzyrice

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