Question

cos π/12 cos 7π/12-sin π/12sin 7π/12

Answer 1

$cos frac{pi}{12}cos frac{7pi}{12}-sinfrac{pi}{12}sin(7pi}{12}=$
$cos(frac{pi}{12}+frc{7pi}{12})=cosfrac{8pi}{12}=$
$cos frac{2pi}{3}=cos(pi-frac{pi}{3})=-cosfrac{pi}{3}=-frac{1}{2}$