[tex]cos(A - B) = cos A cos B + sin A sin B[/tex]
In this case, A is [tex]\frac{\pi }{3}[/tex] and B is [tex]\frac{\pi }{5}[/tex], so:
[tex]cos\frac{\pi}{3} cos\frac{\pi}{5} +sin\frac{\pi }{3} sin\frac{\pi }{5} =cos(\frac{\pi}{3} -\frac{\pi}{5} ) = cos\frac{2\pi}{15}[/tex]
Hope that helps!
