Originally Posted by

**Paymemoney** Hi

I need help the following:

Given $\displaystyle 0 < \alpha,\gamma < \pi$ and $\displaystyle cos(\alpha) = \frac{2}{3}$, $\displaystyle cos(\gamma) = \frac{-1}{3}$ calculate exact expression for $\displaystyle cos(\alpha - \gamma)?$

This is what i have done however it is incorrect:

we know that $\displaystyle cos(\alpha) = \frac{2}{3}, cos(\gamma) = \frac{-1}{3},

sin(\alpha) = \frac{3}{\sqrt{13}} and sin(\gamma) = \frac{-26}{81}$

i sub it into: $\displaystyle cos(\alpha)cos(\gamma) + sin(\alpha)sin(\gamma)$

so i get:$\displaystyle \frac{-\sqrt{13}}{27} + \frac{-78}{81\sqrt{13}}$

P.S