sin95.sin55−cos40/2=?
(sin95.sin55−cos40)/2 mi sin95.sin55−(cos40)/2 mi?
sinx . siny = -1/2[ cos(x+y) + cos(x-y) ] sin95.sin55 = -1/2[ cos150 + cos40] -cos150/2 + cos40/2 - cos40/2 -cos150/2 cos30/2 √3/4