化简cos^2(a+15)+sin^2(a-15))+sin(a+180)cos(a-180)

余弦二倍角公式：Cos2a=1-2Sina^2=2Cosa^2-1正弦二倍角公式：sin2α = 2cosαsinα 所以[2sinacos15°]*[2cosa*sin(-15°)]=(2sinacosa)*(-2sin15°cos15°)=-sin2asin30°=-(1/2)*sin2a看不懂是因为,你给出的解题过程有省略,详细如下：cos^2(a+15)+sin^2(a-15)+sin(a+180)cos(a-180) =cos^2(a+15)+sin^2(a+15)-sin^2(a+15)+sin^2(a-15)+sinacosa =1+[sin^2(a-15)-sin^2(a+15)]+sinacosa =1+[sin(a-15)+sin(a+15)][sin(a-15)-sin(a+15)]+sinacosa =1+[2sinacos15]*[2cosa*sin(-15)]+sinacosa =1-sin2a*sin30+(1/2)*sin2a =1-(1/2)*sin2a+(1/2)*sin2a =1