# Dot Product

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1. Jan 13, 2016

### CheesyPeeps

1. The problem statement, all variables and given/known data

2. Relevant equations
p.q+p.r

3. The attempt at a solution
I've expanded p.(q+r) to give p.q+p.r. The magnitude of p is 3, and since ABE is an equilateral triangle, the magnitude of q is also 3, right? So then p.q=9, but the answer scheme states that p.q=4.5.
I'm still pretty new to mathematics, so I'm probably missing something really obvious, but it's better to make the mistakes now than in my exam!

2. Jan 13, 2016

### Samy_A

p.q is certainly not equal to 9.

Maybe you could start by stating the geometric definition of the dot product in Euclidean space, or $\mathbb R^3$ in this case.
Then you will be able to deduce from that the values of p.q and p.r

3. Jan 13, 2016

### CheesyPeeps

Ah, I've got it now. Thanks for your help!