y=1/(-x^2+2x+3)
y(-x^2+2x+3)=1
yx^2-2yx-3y+1=0
y不=0,则有判别式>=0
即有4y^2-4y(-3y+1)>=0
y^2+3y^2-y>=0
4y^2-y>=0
y(4y-1)>=0
y>=1/4,y