Theorem 1. Let
be continuous on [a, b] and differentiable on (a, b).
is a constant
function if and only if
for all
.
Definition 1. A function
is said to be monotonic increasing (resp. monotonic decreasing)
or simply increasing (resp. decreasing) on an interval
if and only if
, if 
then
(resp.
, if
, then
).
Definition 2. A function
is said to be strictly increasing (resp. strictly decreasing) on an
interval
if and only if
, if
then
(resp.
, if
,
then
).
Theorem 2. Let
be continuous on [a, b] and differentiable on (a, b). Then
(a) if
is strictlyincreasing on [a, b]; and
(b)if
is strictly decreasing on [a, b].
Definition 3. A neighborhood of a point
is an open interval containing
, i.e.
is a neighborhood of
for some
.
Definition 4. A function
is said to attain a relative maximum (minimum) at a point
if
(
) in a certain neighborhood of
, i.e.
such that
(
) for
.