Question

$42 sqrt{6} cos frac{ pi }{4} cos frac{5 pi }{6}$

Answer 1

$42 sqrt{6} cos frac{ pi }{4} cos frac{5 pi }{6} =42sqrt6*frac{sqrt2}{2}*(-frac{sqrt3}{2})=-63\\cosfrac{5pi}{6}=cos(frac{6pi}{6}-frac{pi}{6})=cos(pi-frac{pi}{6})=-cosfrac{pi}{6}$