Hi! I知 novice in the field and surely my question would be very simple for you. I知 now reading a textbook of Mathematics for biologists and try to understand the solved problems in the text. I have trouble with this one:

Find all partial derivatives up to second order of the function: z=(x+y)/(x-y)

The book says:

I am confused with ∂/∂y(∂z/∂x) and ∂/∂y(∂z/∂y). Trying to solve the problem, I get:

I know that for the most functions of two variables ∂/∂y(∂z/∂x) = ∂/∂x(∂z/∂y) and probably the textbook isn稚 wrong, so I would be grateful if you point where I am wrong.

Thank you very much in advance!