Answer
$cos~\frac{\pi}{7}~cos~\frac{\pi}{5}-sin~\frac{\pi}{7}~sin~\frac{\pi}{5}=cos~\frac{12\pi}{35}$
Work Step by Step
$cos(u+v)=cos~u~cos~v-sin~u~sin~v$
$cos~\frac{\pi}{7}~cos~\frac{\pi}{5}-sin~\frac{\pi}{7}~sin~\frac{\pi}{5}=cos(\frac{\pi}{7}+\frac{\pi}{5})=cos(\frac{\pi(5)}{7(5)}+\frac{\pi(7)}{5(7)})=cos(\frac{5\pi}{35}+\frac{7\pi}{35})=cos~\frac{12\pi}{35}$
