(x-1)^2+(y+3)^2=5
所以(2,-1)是圆上一点
圆心(1,-3)与(2,-1)所在直线的斜率=(-3+1)/(1-2)=2
则切线方程的斜率=-1/2
所以切线方程:y+1=-1/2*(x-2)
y=-1/2*x