令a=tanx 则a属于R y=f(x)=(a-a+1)/(a+a+1) ya+ya+y=a-a+1 (y-1)a+(y+1)a+(y-1)=0 a是实数则方程有解 所以判别式大于等于0 (y+1)-4(y-1)>=0 (y+1+2y-2)(y+1-2y+2)>=0 (3y-1)(y-3)