I was reading this article and I'm trying to follow this author's proof. The author jumps from
$$\psi_1(x)\frac{\partial^2\psi_2(x)}{\partial x^2}-\psi_2(x)\frac{\partial^2\psi_1(x)}{\partial x^2}=0$$ to
$$\psi_1(x)\frac{\partial\psi_2(x)}{\partial x}-\psi_2(x)\frac{\partial\psi_1(x)}{\partial x}=Constant$$
I've tried integrating by parts but it just seems to get mucky fast. Can anyone help me clarify how this is such a simple procedure?