This could also be written as

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

This expression means:

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

In our example above, F(x,y) = y + 6 sin x + 5y2, we found

`(delF)/(dely)=1+10y`

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

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

`=del/(dely)[1+10y]`

`=10`