x^2+y^2+kx+2y+1\4k^2+k
=(x+k/2)^2+(y+1)^2+k-1=0
即(x+k/2)^2+(y+1)^2=1-k
方程(x+k/2)^2+(y+1)^2=1-k表示圆
所以 1-k>=0
所以k