(1+2cos^2x-1)/2sinxcosx=3/4
1/tanx=3/4
tanx=4/3
tan2A=2tanA/（1-tanA^2）
2tan(x/2)/(1-tan^2(x/2))=4/3
6tan(x/2)=4-4tan^2(x/2)
2tan^2(x/2)+3tan(x/2)-2=0
(2tan(x/2)-1)(tan(x/2)+2)=0
tan(x/2)=1/2
tan(x/2)=-2