let
x=cosθ ,y=sinθ
(cosθ ,sinθ ) is on x^2+y^2=1
d1=点到x-轴的距离 = sinθ
d2=点到Y-轴的距离 = cosθ
d=点到两坐标轴的距离之和
=d2+d2
=sinθ +cosθ
= √2.sin(θ+ π/4)
max d = √2