- 三角问题设a为第四象限角，若sin3a/sina=13/5,则t
- 设a为第四象限角，若sin3a/sina=13/5,则tan2a等于多少
- sin3a=sin(a+2a)=sinacos2a+cosasin2a
=sina[1-2(sina)^2]+2sina(cosa)^2
=sina-2(sina)^3+2sina[1-(sina)^2]
=3sina-4(sina)^3
sin3a/sina=3-4(sina)^2=13/5
(sina)^2=1/10
sina=-√10/10
cosa=3√10/10
tana=sina/cosa=-1/3
tan2a=2tana/[1-(tana)^2]=-3/4