# Absolute value theorem that I can't convince myself of

1. Sep 16, 2008

### JennyInTheSky

While reading my text, I came across an inequality that I couldn't convince myself of...

For real numbers a,b: $$\left|a+b$$|<= |a|+|b|. Is this something proven? Or is it an axiom or something?

2. Sep 16, 2008

### dodo

3. Sep 16, 2008

### PowerIso

It's a pretty important inequality. I highly suggest you convince yourself of its truth :D

4. Sep 17, 2008

### gunch

To convince yourself of its truth consider what the effect of the signs of a and b have on the inequality.

Case 1( a and b are positive):
|a+b| = a + b = |a|+|b|

Case 2 (a is positive and b is non-positive):
Let b = -y then a and y are positive. If a-y is positive:
$$|a+b|=|a-y| = a-y \leq a \leq |a| + |b|$$
If a-y is non-positive, then y-a is positive and:
$$|a+b|=|a-y| = y-a \leq y = |b| \leq |a| + |b|$$

Case 3 (a and b are negative):
Let a = -x, b = -y:
|a+b| = |-(x+y)| = x+y = |a|+|b|