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Possible Duplicate:
Determinants and volume of parallelotopes

Can you give me a direction about how to prove that $|det(UVW)|$ is the 3D volume of the parallelogram that defined by $U, V$ and $W$, for $U, V, W \in \mathbb{R}^{3}$ ?

Thanks.

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    Not a duplicate as far as I can see. The earlier question asks whether there's a generalization to dimension >3; this one seeks a _proof_ for the 3-dimensional case. An answer to one is not the same as an answer to the other.2011-11-21

1 Answers 1

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First note that $a=\frac{V\times W}{\|V\times W)\|}$ is an unit vector orthogonal to the plane spanned by $V$ and $W$. If $\theta$ is the angle between $U$ and $a$, then $U\cdot a=\|U\|\|a\|\cos\theta=\|U\|\cos\theta$ since $a$ is an unit vector. Notice that $a\cdot U$ is the height $h$ of the parallelepiped formed by $U$, $V$ and $W$ with the parallelogram spanned by $V$ and $W$ as the base (It would be easier to understand if you draw a picture), i.e. $h=a\cdot U.$ On the other hand, the area $A$ of the base is equal to the area the parallelogram spanned by $V$ and $W$, that is $A=\|V\times W\|.$ Therefore, the volume of the parallelepiped is given by $hA=(U\cdot a)\|V\times W\|=U\cdot (V\times W)$ since $a=\frac{V\times W}{\|V\times W)\|}$. Now the result follows from $\det(U,V,W)=U\cdot(V\times W).$

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    Is there a elegant way to expand this prove to $\mathbb{R}^n$?2011-11-20