COS9分之拍減COS9分之2拍加COS9分之3拍減COS9分之4拍
1/2
cosX+cosY=2cos[(X+Y)/2].cos[(X-Y)/2]把COS60放一邊 剩COS20-(COS40+COS80)=COS20-(2COS60COS20)=COS20-COS20=0故答案為2分之ㄧ