Proof of Triangle Inequality and Equality Condition - SEMATH INFO -
For any real vectors $\mathbf{a}$ and $\mathbf{b}$,
holds.
This inequality is called
triangle inequality .
The proof is below.
Proof
Geometrically,
the triangular inequality is an inequality expressing that the sum of the lengths of two sides of a triangle
is longer than the length of the other side as shown in the figure below.
The proof is as follows.
Let $\mathbf{a}$ and $\mathbf{b}$ be real vectors.
The square of the norm of $\mathbf{a} + \mathbf{b}$ satisfies
, where
we used
that the inner product between two vectors is symmetric,
, and
$
\left(\mathbf{a}, \mathbf{b}\right) \leq | \left(\mathbf{a}, \mathbf{b}\right)|
$
generally holds.
Applying the Schwarz inequality,
, to this inequality, we see that
. From this and
, we see that the triangle inequality
holds.
Equality Condition
We will discuss the equality condition of the triangular inequality.
First,
one of the equality condition of the triangle inequality is
$
\mathbf{a} = 0
$
or
$
\mathbf{b} = 0
$
,
because, in this case, it is clear that
. In other cases, that is,
if
$
\mathbf{a} \neq 0
$
and
$
\mathbf{b} \neq 0
$
,
the equality holds if and only if $\mathbf{a}$ and $\mathbf{b}$ are parallel and in the same direction,
, where $t > 0$.
This can be understood by considering as follows.
In the above proof of the triangle inequality,
we only use relations that hold for all vectors other than the Schwarz inequality,
, and the inequality,
.
Therefore,
the equality of the triangular inequality holds if and only if the equality of $(1)$ and that of $(2)$ hold.
We thus consider the equality condition of $(1)$ and $(2)$.
It is known that the equality condtion of Schwarz inequality $(1)$ is
that $\mathbf{a}$ and $\mathbf{b}$ are parallel,
.
Here $t$ can be a posive value or a negative value.
If $t$ is positive,
the equality of $(2)$ holds because
.
On the other hand,
if $t$ is negative,
the equality of $(2)$ does not hold because
.
Therefore,
the equality of $(2)$ holds only if $t$ is positive.
We thus see that
the equality condtion of
$(1)$ and $(2)$ is that
, where $t>0$.
This is the equality condition of the triangle inequality.