设M(x,y)
根号[(x-4)^2+y^2]+根号[(x+4)^2+y^2]=12
根(x^2-8x+16+y^2)=12-根(x^2+8x+16+y^2)
x^2-8x+16+y^2=144-24根(x^2+8x+16+y^2)+x^2+8x+16+y^2
3根(x^2+8x+16+y^2)=18+2x
9(x^2+8x+16+y^2)=324+72x+4x^2
即轨迹方程是：5x^2+9y^2=180