求值：sin21°cos81°-sin69°cos9°
问题描述：
求值：sin21°cos81°-sin69°cos9°
答
sin21cos81-sin69cos9
=-sin21sin69cos81cos9
=[-sin21cos(90-69)][sin(90-81)cos9]
=(-sin21cos21)(sin9cos9)
=-(1/2sin42)(1/2sin18)
=-1/4sin42sin18
额,后面的忘了,自己算算吧,翻翻书,有这种类型的解法的.