I'm asking for a little help.
I'm trying to resolve this derivative $\frac{\partial^2U}{\partial X^2}$ where $u = f(x,y), x = e^s \cos(t) , y = e^s \sin(t)$
Here's how I try to solve it.
$$\frac{\partial U}{\partial X} = \frac{\partial U}{\partial S} \cdot \frac{\partial S}{\partial X}$$
$$\frac{\partial S}{\partial X} = \frac{1}{e^s \cos t}$$
$$\frac{\partial U}{\partial X} = \frac{\partial U}{\partial S} \cdot \frac{1}{e^s \cos t}$$
I'm wondering if $\frac{\partial^2 U}{\partial X^2} = (\frac{\partial U}{\partial S} \cdot \frac{1}{e^s \cos t})'$ and how I could do that.
