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Ok, I have a question that probably has a very simple answer but for some reason I can't see it. Let $a$ and $r$ be two vectors of nonzero length with a common origin and let $\theta$ be the nonzero angle between them. Then, by definition of the cosine funtion, $ \cos \theta = \frac{|a|}{|r|} $ where $|\cdot|$ denotes the norm. On the other hand, the scalar product is given by

$ \langle a, r \rangle = |a|\cdot |r| \cos \theta. $

Putting these facts together we have $ \langle a, r \rangle = |a|\cdot |r| \cdot \frac{|a|}{|r|} = |a|\cdot |a| = |a|^2 = \langle a, a \rangle $ which is a result that is independent of $r$ and thus makes no sense. What is my error?

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Your error is that $\cos(\theta)=\frac{|a|}{|r|}$ is not correct except when $a$ and $r$ are a leg and the hypotenuse of a right triangle, respectively, which is not the case in general.

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    Indeed, that was a very silly error on my part. Thanks.2012-06-23
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Your definition for cosine is wrong. In $\mathbb{R}^2$, let $a = (0,1)$ and $r=(1,0)$ both radiating from the origin. According to your definition, this would give you $cos(\theta) = 1$ even though the angle between these two vectors is $\pi/2$ and $\cos(\pi/2) = 0$.