对方程两边求微分,得
8xdx + 2ydy = 0,
整理,得
dy/dx = -4x/y.
于是,
d^2y/dx^2 = (d/dx)(dy/dx)
= -4[y-x(dy/dx)]/y^2
= -4(y^2-4x^2)/y^3.