圆x^2 + y^2 + 6x +5=0,即 (x + 3)^2+ y^2 = 2^2
圆x^2 + y^2 - 6x -91=0,即 (x - 3)^2+ y^2 = 10^2
设动圆的圆心为O(x,y),据题意,利用动圆心与两个已知圆心的距离关系：
√[(x + 3)^2+ y^2] - 2 = 10 - √[(x - 3)^2+ y^2 ] = 动圆的半径
化简得：3 x^2 + 4 y^2 = 108
即圆心的轨迹方程为椭圆：
x^2 / 36 + y^2 / 27 = 1
圆x^2 + y^2 + 6x +5=0,即 (x + 3)^2+ y^2 = 2^2
圆x^2 + y^2 - 6x -91=0,即 (x - 3)^2+ y^2 = 10^2
设动圆的圆心为O(x,y),据题意,利用动圆心与两个已知圆心的距离关系：
√[(x + 3)^2+ y^2] - 2 = 10 - √[(x - 3)^2+ y^2 ] = 动圆的半径
化简得：3 x^2 + 4 y^2 = 108
即圆心的轨迹方程为椭圆：
x^2 / 36 + y^2 / 27 = 1