[질문] sin20sin40sin80어떻게구해요?

sinasinb=(-1/2){cos(a+b)-cos(a-b)}이다.그러므로sin20sin40=(-1/2){cos60-cos(-20)}=(-1/2){1/2-cos20}=-1/4+(1/2)cos20가된다.또 sin80 =cos10 이므로sin20s