y'=-(2-x)'/(2-x)^2=1/(x-2)^2
切线斜率为：f'(-1)=1/(-1-2)^2=1/9
f(-1)=1/(2-(-1))=1/3
(y-1/3)/(x-(-1))=1/3
切线方程为：y=1/3 x+2/3