- Thread starter
- Banned
- #1

- Thread starter Poirot
- Start date

- Thread starter
- Banned
- #1

- Feb 13, 2012

- 1,704

It is well known that there are two different vector products: the scalar product and the vectorial product. If we hypothise to define a sort of 'inverse vector' we must distinguish between 'scalar inverse' and 'vectorial inverse'. The scalar inverse seems to be reasonably comfortable to manage... the vectorial inverse probably is a little problematic to manage...If P,Q,R,S are 4 points in R^2 then we have the lines PQ and RS. How do we define $\frac{PQ}{RS}$ ?

Kind regards

$\chi$ $\sigma$

Last edited:

- Feb 13, 2012

- 1,704

$\displaystyle X \cdot Y = |X|\ |Y|\ \cos \theta$ (1)

... where $\theta$ is the angle between vectors and |*| is the norm. Let's suppose that the 'scalar inverse' of a vector

$\displaystyle X \cdot X^{-1} = 1$ (2)

It is almost immediate that $X^{-1}$ in that case is not univocally defined because different combinations of $|X^{-1}|$ and $\theta$ can satisfy (2)... it seems that we are not on the right way! ...

Kind regards

$\chi$ $\sigma$

- Thread starter
- Banned
- #4

- Jan 30, 2012

- 2,513

Wikipedia says the following about Menelaus' theorem, which is similar.look up ceva's theorem on wikipedia. That's what I'm on about.

So, it involves ratios of signed segment lengths.This equation uses signed lengths of segments, in other words the length AB is taken to be positive or negative according to whether A is to the left or right of B in some fixed orientation of the line. For example, AF/FB is defined as having positive value when F is between A and B and negative otherwise.

- Admin
- #6

- Jan 26, 2012

- 4,197

Maybe I'm missing something here, but it looks to me like $PQ$ is the length of the vector from point $P$ to point $Q$, and $RS$ is the length of the vector from point $R$ to point $S$. Then the fraction $(PQ)/(RS)$ is normal real division: one length divided by another. $PQ$ is a common, though not, to my mind, the best, notation for the length of that line segment.If P,Q,R,S are 4 points in R^2 then we have the lines PQ and RS. How do we define $\frac{PQ}{RS}$ ?

Is there more context to the problem?

- Jan 30, 2012

- 2,513

- Thread starter
- Banned
- #8

- Admin
- #9

- Jan 26, 2012

- 4,197

If you mean I'm misunderstanding the notation, then I would claim the notation is atrocious. I know for a fact that I have seen this notation used for the simple length of a line segment.Yes this is a simple misunderstanding of notation.

Perhaps you could use something like $\overset{\pm}{\overline{PQ}}$?

- Thread starter
- Banned
- #10