This however is exactly what we need to do the two new derivatives we need above. Both of the first order partial derivatives, and , are functions of x and y and and so we can use (1) to compute these derivatives.
To do this we’ll simply replace all the f ’s in (1) with the first order partial derivative that we want to differentiate. At that point all we need to do is a little notational work and we’ll get the formula that we’re after.