a属于（π，3π/2）第三象限所以sina<0(sina)^2+(cosa)^2=1所以sina=-12/13sin2a=2sinacosa=120/169cos2a=(cosa)^2-(sina)^2=-119/169tan2a=sin2a/cos2a=-120/119tana=-2tan2a=2tana/[1-(tana)^2]=-4/(-3)=4/3
