上下同乘以cosx
y=(sinx+2cosx)/(1-cosx)
利用sinx=2sin（x/2）cos(x/2)
cosx=sin(x/2)^2-cos(x/2)^2
有y=(2sin（x/2）cos(x/2)+2(sin(x/2)^2-cos(x/2)^2))/(sin(x/2)^2+cos(x/2)^2-(sin(x/2)^2-cos(x/2)^2))
化简y=tan(x/2)^2+tan(x/2)-1
令tan(x/2）=t
y=t^2+t-1
t属于全体实数
所以t=-1/2时y取最小值-5/4
所以y>=-5/4
附图一张