Question

(sin 9') * (sin 69') + (sin 81') * (sin21') = ?

(sin 9') * (cos 21') + (cos 9') * (sin 21')

Как дальше решать ? -_-'
( ' - градус) 

Answer 1

(sin 9') * (sin 69') + (sin 81') * (sin21')=(sin 9') * (sin 69') + (cos 9') * (cos 69')=cos(69-9)=cos60=1/2

(sin 9') * (cos 21') + (cos 9') * (sin 21')=sin(9+21)=sin30=1/2