# Math Help - Prove

1. ## Prove

Prove that $\bold{u}_{r} \times (\bold{u}_{r} \times \bold{u}_{\theta}) = - \bold{u}_{\theta}$.

2. Originally Posted by heathrowjohnny
Prove that $\bold{u}_{r} \times (\bold{u}_{r} \times \bold{u}_{\theta}) = - \bold{u}_{\theta}$.
First read the start of this.

Then substitute $\vec{A} = \vec{B} = \bold{u}_{r}$ and $\vec{C} = \bold{u}_{\theta}$ into the formula.

Then note that $\bold{u}_{r}$ and $\bold{u}_{\theta}$ are perpendicular and simplify accordingly.

3. Originally Posted by heathrowjohnny
Prove that $\bold{u}_{r} \times (\bold{u}_{r} \times \bold{u}_{\theta}) = - \bold{u}_{\theta}$.
i assume that they are vectors, butwhat are the elements? (i forgot them already)