# Schwarz Inequality

1. Jun 30, 2015

### ahmed markhoos

Hello,

I'm having a small problem with Schwarz inequality, |u⋅v|≤||u||||v||

the statement is true if and only if cosΘ≤1 !!, I'm familiar with this result but how could it be more than 1?
what is so special in higher dimensions that it gave the ability for cosine to be more than 1? why and how?

2. Jun 30, 2015

### Fredrik

Staff Emeritus
It can't be more than 1. The inequality $|\langle u,v\rangle|\leq\|u\|\|v\|$ holds for all vectors u and v. That's why it makes sense to define "the angle between u and v" as the $\theta$ such that
$$\cos\theta =\frac{\langle u,v\rangle}{\|u\|\|v\|}.$$ This isn't some new version of the cosine function. It's the plain old cosine function that you're familiar with.

Last edited: Jun 30, 2015