x^(2/3) + y^(2/3) = 1
则y^2=(1-x^(2/3))^3=1-3x^(2/3)+3x^(4/3)-x^2
曲线上的点为（x,y）
则到原点的距离：L（x）=√（x^2+y^2)=√[1-3x^(2/3)+3x^(4/3)]
=√[3(x^(2/3)-1/2)+1/4]
当x^(2/3)=1/2时,L（x）min=1/2