(sina+cosa)^2=1/2
所以2sinacosa=-1/2;
sin(2a)=-1/2;
cos(2a=-√3/2;
tan(2a)=sin(2a)/cos(2a)==√3/3
tan(2α)=2tanα/[1-tan^2(α)]
a为钝角
tana=-√3-2;
cos2a=1-2sin^2a sina=
S△ABC=1/2sina AB.AC=
就这些了 基本的运算自己搞定吧