Trivial Inequality
The trivial inequality is an inequality that states that the square of any real number is nonnegative. Its name comes from its simplicity and straightforwardness.
Statement
For all real numbers ,
, with equality if and only if
.
Proof
We proceed by contradiction. Suppose there exists a real such that
. We can have either
,
, or
. If
, then there is a clear contradiction, as
. If
, then
gives
upon division by
(which is positive), so this case also leads to a contradiction. Finally, if
, then
gives
upon division by
(which is negative), and yet again we have a contradiction.
Therefore, for all real
, as claimed.
Applications
The trivial inequality is one of the most commonly used theorems in mathematics. It is very well-known and does not require proof.
One application is maximizing and minimizing quadratic functions. It gives an easy proof of the two-variable case of the Arithmetic Mean-Geometric Mean inequality:
Suppose that and
are nonnegative reals. By the trivial inequality, we have
, or
. Adding
to both sides, we get
. Since both sides of the inequality are nonnegative, it is equivalent to
, and thus we have
as desired.
Problems
Introductory
Find all integer solutions of the equation
.
(No source or solution)
Intermediate
- Triangle
has
and
. What is the largest area that this triangle can have? (Source)