Question

Обчисліть: cos105°+ cos75°

Answer 1

cos 105°+ cos 75°=cos(60+45)+cos(30+45)=cos60cos45-sin60sin45 +cos30cos45-sin30sin45=1/2*√2/2-√3/2*√2/2+√3/2*√2/2-1/2*√2/2=0

cos 105°+ cos 75°=cos(180-75)+cos75=-cos75+cos75=0