`f(x,y,z)=xsin(y+3z)`, then
`(delf)/(delz)=del/(delz)=[xsin(y+3z)]=xdel/(delz)sin(y+3z)`
= `xcos(x+3z)del/(delz)(y+3z)`
=`3cos(y+3z)`.
How to:
to show `(delf)/(delz)` we use \`(delf)/(delz)\` but if we want `del/(delz)` then we don't need pranthesise around
and \`del/(delz)\` will work fine like this: `del/(delz)`