# What does this vector equal to?

1. Apr 27, 2010

### Cuisine123

1. The problem statement, all variables and given/known data
Prove that |$$\vec{A}$$|2+|$$\vec{B}$$|2 equals to $$\frac{1}{2}$$(|$$\vec{A}$$+$$\vec {B}$$|2+|$$\vec{A}$$-$$\vec{B}$$|2)

2. Relevant equations
Perhaps the triangle inequality? I am not sure.

3. The attempt at a solution
I have no idea how to approach this question. Please help me with clear explanations. Thank you.

2. Apr 27, 2010

### tiny-tim

Hi Cuisine123!

Use the definition of modulus: |A + B|2 = (A + B).(A + B) etc

3. Apr 27, 2010

### D H

Staff Emeritus
Perhaps not the triangle inequality. That is an inequality. You are looking for an equality.

Have you tried expanding the right hand side?