# Homework Help: Differentiate the law of Cosine

1. Jan 29, 2008

### ryank614

It is a simple math question, but I am stuck.

The law of cosines is

R^2=h^2 + r^2 - 2 h r cos (theta). Theta is of course the angle facing R.

To differentiate, I first set

r^2 - 2hr cos$$\Theta$$+ (h^2-R^2) = 0

2 r $$\dot{}r$$ - 2 h cos$$\Theta$$ $$\dot{}r$$ + (h^2-R^2) = 0.

Is there any way to put h^2-R^2 in terms of $$\dot{}\theta$$ or d$$\theta$$/dt

2. Jan 29, 2008

### ryank614

Arr, nevermind. I got it.