we can, of course, compute third-,fourth- or even higher-order partial derivative.the
orem 3.1 can be extended to show that as long as the partial derivative are all continuous in an open set, the order of differentiation doesn’t matter. with higher-order partial derivatives notations such as (∂^3 f)/∂x∂y∂x become quite awkward and so, we usually use fxyx
instead,