由f(a)=（2tana）-[（2sin^2a/2 -1）/sin(a/2)cos(a/2)]先对f(a)进行化简f(a)=（2tana）-[-2(1-2sin^2a/2 )/2sin(a/2)cos(a/2)]=(2tana)+2cosa/sina=2(sina/cosa+cosa/sina)=2[(sin^2a+cos^2a)/sinacosa...