(3) Differentiate .
This may be written:
The vanishes, and we have
;
or,
;
or,
.
(4) Differentiate
A direct way of doing this will be explained later (see p.67); but we can nevertheless manage it now without any difficulty.
Developing the cube, we get
;
hence
.
(5) Differentiate
;
or, more simply, multiply out and then differentiate.