Proof of triangle inequality

Question

I understand intuitively that this is true, but I'm embarrassed to say I'm having a hard time constructing a rigorous proof that $|a+b| \leq |a|+|b|$. Any help would be appreciated :)

Answer 1

From your definition of the absolute value, establish first $|x| = \max\{x,-x\}$ and $\pm x ≤ |x|$.

Then you can use \begin{align*} a + b &≤ |a| + b ≤ |a| + |b|,\quad\text{and}\\ -a - b &≤ |a| -b ≤ |a| + |b|. \end{align*}

Answer 2

$$|a|^2+|b|^2+2|a||b|\geq a^2+b^2+2ab$$ $$(|a|+|b|)^2 \geq (a+b)^2\phantom{a}(\because \forall x\in \mathbb{R};\phantom{;}x^2=|x|^2)$$

$$||a|+|b||\geq |a+b|$$

$$\therefore |a|+|b|\geq |a+b|$$

Answer 3

If a neat algebraic argument does not suggest itself, we can do a crude argument by cases, guided by the examples $a=7,b=4$, $a=-7,b=-4$, $a=7, b=-4$, and $a=-7, b=4$.

If $a\ge 0$ and $b\ge 0$ then $|a+b|=|a|+|b|$.

If $a\le 0$, and $b\le 0$, then $|a+b|=-(a+b)=(-a)+(-b)=|a|+|b|$.

Now we need to examine the cases where $a$ is positive and $b$ is negative, or the other way around. Without loss of generality we may assume that $|b|\le |a|$.

If $a\gt 0$, then $|a+b|=|a|-|b|$. This is $\lt |a|$, and in particular $\lt |a|+|b|$.

If $a\lt 0$, then again $|a+b|=|a|-|b|$.

Answer 4

Since the proof of CW is necessary and similar I will do both

1. Start both proofs with the fact that a vector dotted with itself is greater than or equal to 0
2. for CW substitute vector = x-ty, for triangle inequality vector = x+y
3. for CW, after dotting x-ty with itself let t = (x.y)/(y.y), for triangle ineq. after dotting x+y with itself and getting a quadratic equation with a dot product in the middle, use CW to show that this quadratic is less than or equal to the same quadratic with the moduluses of the vectors of the dot product in the equation on the left
4. after rearranging both sqrt both sides

Answer 5

These kind of questions require clarification about the assumptions one begins with. This answer covers the whole package often referred to as "the triangle inequality". I make only the following assumptions, which (with the set $\mathbb R$ of real numbers in place of set $X$) are part of any axiomatization of the real number system:

• There is a commutative group operation $(a,b) \mapsto a+b$ with neutral element $0$ in the set $X$.
• There is a linear order $<$ in the set $X$, compatible with $+$ in the sense that if $a<b$, then $a+c<b+c$.
• The absolute value $|x|\in X$ of $x\in X$ is $-x$ in the case of $x<0$, and $x$ in the cases of $x=0$ and $0<x$.

Note that if $x\neq 0$, then either $x<0$, whence $0<-x=|x|$, or $0<x$, so $0<|x|$ in this case as well. If $|x|=0$, then neither $x<0$ nor $0<x$, since each of these inequalities implies $0<|x|$. These observations prove the following basic result, which is used below, and in which $a\leq b$ stands for "$a<b$ or $a=b$''.

(Positive definiteness.) For each $x\in X$ the inequality $0\leq|x|$ holds, and $0=|x|$ if and only if $x=0$.

Remark. The formal claim $x\leq y$ is also called "inequality". By convention the name "strict inequality" is reserved for the formal claim $x<y$ to underline that it actually claims that the equality $x=y$ does not hold.

The proof of the basic fact that $|-x|=|x|$ holds for every $x\in X$, which I also use below, is immediate:

1. If $x<0$, then $0<-x$, so $|-x|=-x=|x|$.
2. If $0<x$, then $-x<0$, so $|-x|=-(-x)=x=|x|$.
3. If $x=0$, then $-x=x$, so $|-x|=|x|$.

The triangle inequality regarding addition $+$ often refers to the combination of the following two non-strict inequalities together with the statements below them, that clarify when these inequalities are strict:

$$ \big||a|-|b|\big| \leq |a-b| \leq |a|+|b| $$

• The left inequality is strict if and only if either $a<0<b$ or $b<0<a$.
• The right inequality is strict if and only if either both $a<0$ and $b<0$, or both $0<a$ and $0<b$.

This more precise formulation of "the triangle inequality" has the following elementary proof by cases:

1. Case of $0<a$ and $0<b$. In this case clearly $\big||a|-|b|\big|=|a-b|$. Furthermore,

   • If $a<b$, then $a-b<0$, so $|a-b|=-(a-b)=-a+b=-|a|+|b|<0+|b|<|a|+|b|$.
   • If $b<a$, then $0<a-b$, so $|a-b|=a-b=|a|-|b|<|a|+0<|a|+|b|$.
   • If $a=b$, then $a-b=0$, so $|a-b|=0+0<|a|+|b|$.

2. Case of $a<0$ and $b<0$. Apply case 1 for $-a$ and $-b$ in place of $a$ and $b$, and use $|-x|=|x|$.

3. Case of $0<a$ and $b<0$. Now $0<a-b$, so $|a-b|=a-b=a+(-b)=|a|+|b|$.

   • If $a<-b$, then $|a|<|b|$, so $|a|-|b|<0$, and therefore $$\big||a|-|b|\big|=-(|a|-|b|)=-(a-(-b))=-a-b<0-b<a-b=|a-b|.$$
   • If $-b<a$, then $|b|<|a|$, so $0<|a|-|b|$, and therefore $$\big||a|-|b|\big|=|a|-|b|=a-(-b)=a+b<a+0<a-b=|a-b|.$$

4. Case of $a<0$ and $0<b$. Apply case 3 for $-a$ and $-b$ in place of $a$ and $b$, and use $|-x|=|x|$.

5. Case of $a=0$. In this case $\big||a|-|b|\big|=\big|0-|b|\big|=\big||b|\big|=|b|$, and since $$|b|=|-b|=|0-b|=|a-b| \qquad \text{and} \qquad |b|=0+|b|=|0|+|b|=|a|+|b|,$$ we have $\big||a|-|b|\big|=|a-b|=|a|+|b|$.

6. Case of $b=0$. Apply 5, switching $a$ with $b$, and use $|-x|=|x|$.

These cases exhaust all the possibilities and are mutually exclusive.

Remark. Instead of the rather cumbersome wording above, the phrases "$a$ and $b$ are of the same sign" and "$a$ and $b$ are of different signs" are commonly used. Then "the sign of $x$" is $+$ in case of $0<x$, and $-$ in case of $x<0$, while the neutral element $0\in X$ is considered as having no sign at all.

I cannot see any significant short-cut in proving the claim above, provided perhaps by some algebra-trick. (There is not much algebra in a linearly ordered commutative group.) We are so accustomed to using triangle inequality while "computing stuff" by applying various algebraic identities, that it is easy to forget that there is logic involved in the triangle inequality. In logic one sometimes just needs to comb through all the cases.

The significance of the triangle inequality is not in some deep insight its proof requires, but rather in its usefulness and in its elegant formulation compared to the tedious case-by-case proof it seems to require.