x平方+y平方-4x+2y-4=0
(x-2)^2+(y+1)^2=9
所以:圆心(2,-1),半径=3
A到园心的距离=((3-2)^2+(2-(-1))^2)^(1/2)=10^(1/2)
切线长^2=(10^(1/2))^2-3^2=1
切线长=1