由3sina=4cosa得tana=4/3
所以tana=2tan(a/2)÷{1-[tan(a/2)]^2}=4/3
2-2[tan(a/2)]^2-3tan(a/2)=0
tan(a/2)=-2或tan(a/2)=1/2
又sina=2tan(a/2)÷{1+[tan(a/2)]^2}