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.

The answer is 1-radical 2

Whats my error?

\(\displaystyle r = 8(1 + \cos{\theta})\)

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

So when \(\displaystyle \theta = \frac{\pi}{4}\)

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

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

\(\displaystyle = -4\sqrt{2}\).