令a=tanx
则a属于R
y=f(x)=(a²-a+1)/(a²+a+1)
ya²+ya+y=a²-a+1
(y-1)a²+(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)²-4(y-1)²>=0
(y+1+2y-2)(y+1-2y+2)>=0
(3y-1)(y-3)