Volume of a pyramid as a determinant?

linear-algebra determinant

I have three given points, A, B and C, each of them is a corner of a pyramid. Another corner is located in origo.

The task is to set up a determinant to describe the pyramids volume.

Unfortunately, my book and Wikipedia won´t agree on how to do this, that´s why I´m asking you guys.

PS. The follow up question is if the volume would be any different if the position vectors (a, b and c) and origo all where located in the same plane?

Any help would be very appreciated!


Solution 1:

The parallelepiped spanned by $\mathbf a, \mathbf b, \mathbf c$ has (oriented) volume $(\mathbf a\times \mathbf b)\cdot \mathbf c$ (or with any permutation thereof). The pyramid has $\frac16$ of this volume. The expression can also be written as $$V = \frac16 \det(\mathbf a, \mathbf b, \mathbf c)$$ i.e. one sixth of the determinant of the matrix made from the three given vectors. If all vertoces of the pyramid are coplanar, the volume is obviously 0 (and that is also what the determinant gives).

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