Question

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

Answer 1

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