- #1
mrneglect
- 11
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Vector Planes & Orthogonality -- Help!
I must be doing something really stupid, and I'll kick myself when you point it out, but I'm having difficulty with this question:
Find the unit normal to the plane x + 2y – 2z = 15. What is the distance of the plane from the origin?
OK, so I know I need three points in the plane to identify it. I found points:
A = (15,0,0)
B = (15,1,1)
C = (13,2,1)
They all seem to satisfy the equation, right?
So now I find two nonparallel vectors in this plane:
A->B = 0i + 1j + 1k
A->C = -2i + 2j + 1k
They do indeed join those points, right?
So now I want to find an orthogonal vector, so I take the cross product by calculating the determinant of the 3x3 matrix:
[i j k]
[0 1 1]
[-2 2 1]
= (1-2)i - (-2-0)j + (0+2)k
= -i + 2j + 2k
This vector ought to be orthogonal to vectors A->B, A->C and B->C (and their negatives). But I try to test it with the usual method of adding up the parts and seeing if they result in zero, using A->C as my comparison vector:
(-2 * -1) + (2 * 2) + (1 * 2) = 8
Why doesn't this equal zero? I have a feeling that the middle (j) term is wrong, because if that was (2 * -2) then the whole thing would equal zero, but I don't see where I've got my signs wrong in the method.
Any help would be much appreciated.
Cheers!
I must be doing something really stupid, and I'll kick myself when you point it out, but I'm having difficulty with this question:
Find the unit normal to the plane x + 2y – 2z = 15. What is the distance of the plane from the origin?
OK, so I know I need three points in the plane to identify it. I found points:
A = (15,0,0)
B = (15,1,1)
C = (13,2,1)
They all seem to satisfy the equation, right?
So now I find two nonparallel vectors in this plane:
A->B = 0i + 1j + 1k
A->C = -2i + 2j + 1k
They do indeed join those points, right?
So now I want to find an orthogonal vector, so I take the cross product by calculating the determinant of the 3x3 matrix:
[i j k]
[0 1 1]
[-2 2 1]
= (1-2)i - (-2-0)j + (0+2)k
= -i + 2j + 2k
This vector ought to be orthogonal to vectors A->B, A->C and B->C (and their negatives). But I try to test it with the usual method of adding up the parts and seeing if they result in zero, using A->C as my comparison vector:
(-2 * -1) + (2 * 2) + (1 * 2) = 8
Why doesn't this equal zero? I have a feeling that the middle (j) term is wrong, because if that was (2 * -2) then the whole thing would equal zero, but I don't see where I've got my signs wrong in the method.
Any help would be much appreciated.
Cheers!
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