del
You do realise that your formatting is messed up, right?
From context, you mean
Given that $\displaystyle \cos(A \pm B) = \cos A\cos B \mp \sin A\sin B$
prove that $\displaystyle \cos(90^\circ - \theta) = \sin\theta$.
So let $\displaystyle A = 90^\circ$ and $\displaystyle B = \theta$ and write it out..