已知sin(a+β)=3/5，cos(a-β)=-12/13且π/2 sin2a=sin[(a+β)+(a-β)] =sin(a+β)•cos(a-β)+cos(a+β)•sin(a-β)． 因为sin(a+β)=3/5，π/2
已知sin(a+β)=3/5，cos(a-β)=-12/13且π/2 sin2a=sin[(a+β)+(a-β)] =sin(a+β)•cos(a-β)+cos(a+β)•sin(a-β)． 因为sin(a+β)=3/5，π/2
