x的平方+y的平方-4x+2y-4=0(x-2)^2+(y+1)^2=9即圆心坐标O（2,-1）,半径R=3设中点M坐标是：M（x,y)|OM|^2=(X-2)^2+(y+1)^2根据“勾股定理”得：OM^2+(2/2)^2=R^2(x-2)^2+(y+1)^2+1=9即中点方程是：(x-2)^2+(y+1)^2=8...