cosa=-4/5,a属于(π/2,π)sina=3/5sin2a=2sinacosa=-24/25tana=-4/3=2tan(a/2)/[1-tan^2(a/2)]-2[1-tan^2(a/2)]=3tan(a/2)2tan^2(a/2)-3tan(a/2)-2=0[2tan(a/2)+1][2tan(a/2)-2]=02tan(a/2)-2=0tan(a/2)=1