已知π/2
问题描述：
已知π/2
答
应该是cos(α+β)=-4/5
π/2π0cos( α-β)=12/13==> sin( α-β)=5/13,
cos(α+β)=-4/5==>sin(α+β)=-3/5
sin2α
=sin(α+β + α-β)
=sin(α+β)cos(α-β)+cos(α+β)sin(α-β)
=-3/5*12/13 -4/5*5/13
=-56/65
cos2β
=cos(α+β - α-β)
=cos(α+β)cos(α-β)+sin(α+β)sin(α-β)
=-4/5*12/13 -3/5*5/13
=-63/65