# Calculus question

1. May 27, 2008

### raintrek

1. The problem statement, all variables and given/known data

I have the following expression:

$$cos\theta = \frac{\vec{a}\cdot \vec{b}}{ab}$$
(this is simply taken from a dot product rule for two vectors)

However I need to find $$\nabla_{\vec{r}i} \theta$$

Is there a way I can do it without involving differentials of arccos and the like? I can't use a small angle approximation as these angles are around 60-100 degrees. Any suggestions appreciated!

2. May 27, 2008