Sin9°*cos99°-sin99°*cos9° помогите

Question

Sin9°*cos99°-sin99°*cos9° помогите

Answer 1

Sin9*Cos(90-9) - Sin(90-9)Cos9 = Sin9*Sin9 - Cos9Cos9 = Sin^2 9 -Cos^2 9 = - Cos18