(2) We use notation
.
Clearly,
has to lie on the boundary of
.
Also, the
has to satisfy the
condition
where the
is taken among all directions such that
remain in
for small
.
The differentiation reveals
that
For any
the difference
is a valid
.
Hence, the (2) follows.

(3) Since
we can write from
(2)
We add the above and
obtain
Hence,