x^2+y^2=2x
(x-1)^2+y^2=1
圆心(1,0)半径1
过(-2,0)作圆的切线,切线与x轴夹角为a
sina=1/3
tana=根号2/4
所以斜率的取值范围是(-根号2/4,根号2/4)