Let f(x, xy) be f( 1, 2), for example, f' 1 represents the first derivative of x, and f'' 12 represents the first derivative of x, and then the derivative of xy, so as to facilitate the later operation, and the textbook is also written in this way, which means that there is no problem.

We know the law of derivative of composite function, and here we actually use the law of derivative of composite function to find it. If not, I am not used to using these symbols to simplify the expression. Here are the solutions:

△ 2 * z/△ x △ y = (△ z/△ x) * (△ z/△ y). Now let's find the partial derivative of x first, and then the partial derivative of y..

△z/△x=f' 1+(f'2)*y

△^2*z/△x△y=xf'' 12+f'2+xyf''22