已知cos(π-a)*cos(3π/2+a)=-1/8,且π/4

问题描述:

已知cos(π-a)*cos(3π/2+a)=-1/8,且π/4

cos(π-a)*cos(3π/2+a)=-1/8 ==>-cosa sina=-1/8 ==>sina*cosa=1/8 ==>2sina*cosa=1/4 ==>sin(2a)=1/4 因为π/4