Question

cos(40)cos(20)-sin(40)sin(20)

Answer 1

По тригонометрическому свойству:
$cos(alpha + beta)=cosalpha cdot cosbeta - sinalpha cdot sinbeta$
$cos40^{circ} cdot cos20^{circ}-sin40^{circ} cdot sin20^{circ} = cos(40^{circ}+20^{circ})=cos60^{circ}= frac{1}{2}$