Man my professor in UIC prof. Saeed in the ME department doesn't know how to explain shit like you. He makes it complicated as hell and the way he explains it is work for 200 years ago not for today. Thanks a lot
When you pulled the A out, the A could depend on what position x you are at though, in which case it cannot be taken out of the partial derivative, in fact pulling it out is making an assumption of constant cross section in the yz plane, at the very least this assumption should be stated
