2sinxcosx/2cos^2x=3/4tanx=3/4tan2A=2tanA/（1-tanA^2）3/4=2tan(x/2)/(1-tan^2(x/2))3-3tan^2(x/2)=8tan(x/2)3tan^2(x/2)+8tan(x/2)-3=0(3tan(x/2)-1)(tan(x/2)+3)=0tan(x/2)=1/2tan(x/2)=-3