x^2+y^2+4x+2by+b^2=0(x^2+4x+4)+(y^2+2by+b^2)=4(x+2)^2+(y+b)^2=4圆咐悔心坐标衡陪正为(-2,-b),半径为2圆乱缺x轴相切∴|b|=2∴b=±2
