切点的轨迹方程: x^2+2y^2-2x-4y=0;对 x2/12+y2/6=k求导,得x/6+2yy'/6=0,y'=-x/2y,椭圆上一点(x0,y0),则(yo-2)/(x0-2)=y0'=-x0/2y0;(斜率相等),化简得 x^2+2y^2-2x-4y=0 轨迹方程