Differentiation Related rates of change

[DanteNZ]

Stuck on how to do this, hope someone can help.

Q)

The radius, r, of a circle is changing at a rate of -2 cm/sec.
What is the rate of change of the circumference, C?

A)

I have messed up somewhere big...
This is what I come up with.

dC/dt = dC/dr *(multiply) dr/dt
For some reason my brain wants me to derive the formula of a circumference. But I can't.

Thanks in advance.

 

[tkhunny]

Circumference = C(t) = 2pi(r(t))

dC/dt = 2pi(dr/dt)

dr/dt = -2 cm/sec

Substitute.