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## Homework Statement

I have the following expression:

[tex]cos\theta = \frac{\vec{a}\cdot \vec{b}}{ab}[/tex]

(this is simply taken from a dot product rule for two vectors)

However I need to find [tex]\nabla_{\vec{r}i} \theta[/tex]

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!