点（x',y' ）到 m(2,4)等于令一条切线长即为4(2-x')^2+(4-y')^2=16x^2+y^2-4x-8y+8=0 ( x2+y2=4)x'=2-2y' 带入x2+y2=4得x'=-6/5 y'=8/5直线AB方程为y=-1/2(x-2)令x=0 y=1令y=0 x=2故b=1a=2椭圆t:x2/2^2+y2=1...