椭圆的参数方程：x=4cosa y=2sina
点到直线x+2y-根号2=0的距离d=|4cosa+4sina-√2|/√5
=|4√2sin(a+π/4)-√2|/√5 当2sin(a+π/4)=-1时
最大值=|-4√2-√2|/√5
=√10