# Extension of the Triangle Inequality

1. Oct 8, 2010

### Caspian

Pretty much knows the triangle inequality.
$$\left| a + b \right| \le \left| a \right| + \left| b \right|$$

I was reading a source which asserted the following extension of the triangle inequality:
$$\left| a + b \right|^p \le 2^p \left(\left| a \right|^p + \left| b \right|^p\right)$$

This is bugging me because I can't figure out how to prove or disprove it. It's sensible enough that it might actually be true... but want to know for sure whether it's true.

I tried proving it by induction, but it got really messy...

Is this inequality true? Or, is it wrong? Thanks!

2. Oct 8, 2010

### joshyxc1979

when you tried induction, did you try the binomial theorem?

3. Oct 9, 2010

### losiu99

It follows from convexity of xp for p > 1.
$$(\frac{1}{2}|a|+\frac{1}{2}|b|)^p\leq \frac{1}{2}|a|^p+\frac{1}{2}|b|^p$$
$$(|a|+|b|)^p\leq 2^{p-1}(|a|^p+|b|^p)$$