1. The problem statement, all variables and given/known data in the notes , 'by applying chain rule to LHS of the above equation ' , which equation is the author referring to ? it's given that f /x + (f/z)(z/x) = 0 , As we can see , the function contain variable x , y and z 2. Relevant equations 3. The attempt at a solution why not f /x + (f/z)(z/x) +(f/y)(y/x) = 0 ???