Proving 3 vectors are coplanar

1. Feb 11, 2007

tiagomendes

Prove that the vectors a=3i+j-4k
b= 5i-3j-2k
c= 4i-j-3k are COPLANAR

2. Relevant equations

(axb)c=0

3. The attempt at a solution

If (axb)c=0 then c is orthogonal to axb and therefore c is in the plane perpendicular to axb since axb is perpendicular to both a and b, both a,b,c are in the same plane perpendicular to axb.
My problem is that my answer when using the formula doesnt equal 0 meaning that it isn't coplanar which means its wrong because i have to prove it is. Can someone show me a walkthrough and how to use this formula because im doing something wrong

2. Feb 11, 2007

Another approach would be to test if they are linearly independent. If they are not, then they are coplanar.

3. Feb 11, 2007

tiagomendes

Thank you ill have a go at it... And i just realized something... this is in the wrong forum right?

4. Feb 11, 2007

Actually, it isn't.

5. Feb 11, 2007

tiagomendes

hmm then it means there is a mistake in my textbook...
Grrr i hate when tht happens so i wasn't misusing the formula god :P just wasted 3 hours of my time but atleast i know the problem :)

6. Feb 11, 2007

tiagomendes

oh wait... i completely misread... i understood you said it isnt as in the question isnt coplanar :P lol sorry

7. Feb 11, 2007

The question states that you have to check if the vectors ARE coplanar, right? But it doesn't really matter, since all you need to know is that, as I already wrote, if they are linearly independent then they AREN'T coplanar. If they, of course, happen to be linearly dependent, then they ARE coplanar.

8. Feb 11, 2007

tiagomendes

I have been looking on the internet to find methods on how to work out linear independency but i haven't found any, any that i understand.
do you happen to know any tutorials for this that are simple to understand

9. Feb 11, 2007

Hootenanny

Staff Emeritus
One method (the one I find easiest) is to place the three vectors into a 3x3 matrix and find the determinant. If the determinant is non-zero then the vectors are linearly independent. This is equivalent to using the formula you originally posted.

10. Feb 11, 2007

All you need is the definition of linear independency of a set of vectors. After you find it, you can create an equation from your vectors and the solution of this equation will tell you everything about their dependency/independency, i.e. if they're coplanar or not.

Edit: what Hootenanny suggested is something that you'll need to solve after developing the vector equation (i.e. the system of equations) I was talking about.

11. Feb 11, 2007

tiagomendes

I dont know how to use Matrixs because i havent really learnt about them... But considering My problem how would i input the data into this formula (axb)c=0 my answer always gives something else :S

12. Feb 11, 2007

You don't need to use matrices, it's just a formality.

OK, you have three vectors, a, b, and c. They are linearly independent if the equation

a x + b y + c z = 0 implies x = y = z = 0, where x, y and z are scalars, i.e. real numbers in this case. Can you set up that equation and try to solve it for x, y, and z (or call them whatever you like)?

13. Feb 11, 2007

tiagomendes

Ill have a think about it ill get back to you later and thank you :)

14. Feb 11, 2007

tiagomendes

Isnt it true that A set of points is said to be COPLANAR if and only if they lie on the same geometric plane THREE points are ALWAYS COPLANAR.
Could i use this statement to answer the question?

15. Feb 11, 2007

rdt2

If the three vectors are coplanar then the volume of the parallelepiped spanned by the vectors will be zero. This volume is given by the vector triple product, so the vector triple product - as given by your formula - will be zero.

It is.

http://en.wikipedia.org/wiki/Parallelepiped

16. Feb 11, 2007

Hootenanny

Staff Emeritus
You don't have three points, you have three lines.

17. Feb 11, 2007

HallsofIvy

Staff Emeritus
I'm coming in late to this but I just did a quick calculation of (axb).c and it DOES in fact, equal 0. If you are still having difficulty, show us exactly what you did.

As Hootenanny said, the simplest way to do the triple product is to use the three vectors as the rows of a single determinant. That should be 0.

18. Feb 11, 2007

tiagomendes

Re:

Could you tell me which values you put in the equation to help me understand it?

19. Feb 11, 2007

Hootenanny

Staff Emeritus
Okay, I am assuming you know how to calculate 2x2 determinants?

20. Feb 11, 2007

HallsofIvy

Staff Emeritus
YOU said you knew how to do this but were just getting the wrong result! Are you now saying you have no idea how to set it up?

You said the vectors were a=3i+j-4k, b= 5i-3j-2k, c= 4i-j-3k .
The cross product of a and b is, of course,
$$\left|\begin{array}{ccc} i && j && k \\ 3 && 1 && -4 \\ 5 && -3 && -2\end{array}\right|$$
and you want to take the dot product of that with 4i- j- 3k.

But since the dot product would just multiply corresponding components, that is, 4 times the i component, -1 times the j component and -3 times the k component, that is exactly the same as expanding
$$\left|\begin{array}{ccc} 4 && -3 && k \\ 3 && 1 && -4 \\ 5 && -3 && -2\end{array}\right|$$
by the first row.

Is that what you did? What did you get?