x² + y² -4x - 2y + 4 = 0
(x-2)² + (y - 1)² = 1
此圆的圆心为A(2, 1), 半径为r = 1
所求的圆与直线y=0即x轴相切,而且半径为R=4,圆心B的纵坐标肯定为±4
(1) 圆心B的纵坐标为4
设B(b, 4)
二圆外切,则|AB| = R+r
|AB|² = (R+r)²
(b-2)²+(4-1)² = (4+1)²
(b-2)² = 16
b = 6 或b = -2
B(6, 4)或(-2, 4)
圆方程为:(x-6)² + (y - 4)² = 16
或(x+2)² + (y - 4)² = 16
(2) 圆心B的纵坐标为-4
设B(b, -4)
二圆外切,则|AB| = R+r
|AB|² = (R+r)²
(b-2)²+(-4-1)² = (4+1)²
b = 2
B(2, -4)
圆方程为:(x-2)² + (y + 4)² = 16