# Homework Help: Very Quick Differentiation Question

1. Aug 2, 2011

### ozone

First I attempted to ipmlicitly differentiate
c2 = a2 + b2 -2abcos(theta)

da/db

I almost got the correct solution according to MIT's key, however I missed one part. The cos from the original function remained cos, and I had differentiated it to -sin.

I'm just wondering why it remained cos instead of being differentiated?

2. Aug 2, 2011

### SteamKing

Staff Emeritus
Unless theta is a function of b, then cos(theta) is a constant when trying to find the derivative with respect to b.

3. Aug 3, 2011

### HallsofIvy

It's very difficult to read what you have written. Are those "2"s after the letters squares? If you cannot use use LaTeX, $c^2= a^2+ b^2- 2ab cos(\theta)$, at least use "^" to indicate powers: c^2= a^2+ b^2- 2ab cos(theta).

But the real question is "differentiate what variable with respect to what other variable?"

4. Aug 3, 2011

### ozone

Thanks, but SteamKing already helped solve my problem.. Sorry I will be sure to start using the forums code based mathematical notation from now on.