I cannot understand the following process in the textbook. Could you help me to understand it?
$$\frac{\partial}{\partial x} = \cos\theta\frac{\partial}{\partial r} -\frac{\sin\theta}{r}\frac{\partial}{\partial\theta}$$
$$\frac{\partial^2}{\partial x^2} = \left[\cos\theta\frac{\partial}{\partial r} -\frac{\sin\theta}{r}\frac{\partial}{\partial\theta}\right]^2 \\=\cos^2\theta\frac{\partial^2}{\partial r^2}-2\left(\frac{\sin\theta \cos\theta}{r}\right)\frac{\partial^2}{\partial r \partial \theta}+\frac{\sin^2\theta}{r^2}\frac{\partial^2}{\partial \theta^2}+\frac{2\sin\theta \cos\theta}{r^2}\frac{\partial}{\partial \theta} +\frac{\sin^2\theta}{r}\frac{\partial}{\partial r}$$