This could also be written as

`del/(delx)[[delF)/(delx)]`

This expression means

Find the partial derivative with respect to x of the partial derivative with respect to x.

In our example above, we found

`(delF)/(delx)=6\ cos\ x`

To find `(del^2F)/(delx^2)`, we need to find the derivative with respect to x of `(delF)/(delx)`.

`(del^2F)/(delx^2)=del/(delx)[[delF)/(delx)]`

`=del/(delx)[6\ cos\ x]`

`=-6 sin x`