# Homework Help: Perpendicular vectors, triangle, tetrahedron

1. Feb 8, 2012

### furor celtica

Prove that, if (c - b).a = 0 and (c - a).b = 0, then (b - a).c = 0. Show that this can be used to prove the following geometric results:
a. The lines through the vertices of a triangle ABC perpendicular to the opposite sides meet in a point.
b. If the tetrahedron OABC has two pairs of perpendicular opposite edges, the third pair of edges is perpendicular.
Prove that also, in both cases, (OA)^2 + (BC)^2 = (OB)^2 + (CA)^2 = (OC)^2 + (AB)^2

3. The attempt at a solution

So for the very first task I proceeded by attributing coordinates to each vector: a = (x(1), y(1), z(1)), b = (x(2), y(2), z(2), c = (x(3), y(3), z(3)), where the numbers are actually at the bottom left of the coordinates, but I don't know how to use that notation here.
Anyway I could post all my work but it would take a long time; I didn't have much of a problem at all with the first task and proved that (b - a).c = 0. It took a while though, as you can imagine; is there another way to solve problems like this, or does one always have to use attributed coordinates and deal with those?

With a. and b. I'm stuck, however.
a. I usually would try to solve a question like this by taking into account the end result and what exactly I'm supposed to end up with (and often work backwards to return to the given results), but here I'm not sure at all what you end up with algebraically when three vectors meet. What am I looking for? Also I'm confused on how to find the vectors that are perpendicular to the opposite sides, how to formulate them with regard to the coordinates of A, B and C.
b. Here I immediately became confused as to what is meant by 'opposite edges'. But besides that it seems to be a fairly straightforward variation of the very first task, am I correct?
The last task seems simple as well, as I would just take the attributed coordinates and reformulate the magnitudes of the given vectors to be equivalent, perhaps with some help from the result in the first task. I haven't tackled this one yet.

2. Feb 8, 2012

### tiny-tim

hi furor celtica!
no!! never use coordinates to solve vector problems unless you have to!

the whole point of vector notation is that it makes things easier!

in this case, use the distributive rule
hint: choose a suitable origin

3. Feb 8, 2012

### furor celtica

how should i solve the first task then?

4. Feb 8, 2012

### tiny-tim

use the distributive rule

5. Feb 9, 2012

### furor celtica

haha ok i feel silly now. i'm still stuck on a., though.

6. Feb 9, 2012

### tiny-tim

he he
hint: (c - b).(a - 0)

7. Feb 9, 2012

### furor celtica

really not seeing it! what do you mean by 'suitable origin'? And what is the end result i'm looking for, exactly?

8. Feb 9, 2012

### tiny-tim

what is the geometric meaning (in words) of (c - b).(a - 0) = 0 ?

9. Feb 10, 2012

### furor celtica

vector BC is perpendicular to the position vector of A

10. Feb 10, 2012

### tiny-tim

ok, and how would you choose the position of O so as to apply that to the question?

11. Feb 13, 2012

### furor celtica

take O and A as the same point?

12. Feb 13, 2012

### tiny-tim

how does that help?

you want OA to be a line

13. Feb 13, 2012

### furor celtica

gnaaargh its your fault being all mysterious. i really don't see it man, i've been going over this question for ages so i'm probably missing the really obvious, but could you just be a bit clearer?

14. Feb 13, 2012

### tiny-tim

how would you use vectors to define the line through vertex A perpendicular to the opposite side, BC?