# Homework Help: If certain entries of this matrix are all nonzero, show that the only

1. Feb 25, 2013

### s3a

1. The problem statement, all variables and given/known data
Matrix U is the following matrix:
http://www.wolframalpha.com/input/?i={{a,b,c},{0,d,e},{0,0,f}}

Question:
If a, d, f are all nonzero, show that the only solution to Ux = 0 is x = 0. Then the upper triangular matrix U has independent columns.

The solution says:
xcomplete = xparticular + xhomogeneous = (1/2,0,1/2,0) + x2 (-3,1,0,0) + x4(0,0,-2,1)

2. Relevant equations
xcomplete = xparticular + xhomogeneous

3. The attempt at a solution
I'm very confused; I'm not sure where to start.

Any help in figuring out what I need to do would be really appreciated!

2. Feb 25, 2013

### Dick

Re: If certain entries of this matrix are all nonzero, show that the o

Write out the equations that correspond to Ux=0, putting x=(x1,x2,x3). I think the solution you quoted is for a different problem. U is 3x3 and the solution has four dimensional vectors in it.

3. Feb 25, 2013

### s3a

Re: If certain entries of this matrix are all nonzero, show that the o

Oh, oops.

Ux = 0

{{a,b,c},{0,d,e},{0,0,f}} {{x_1},{x_2},{x_3}} = {{0},{0},{0}}

I can see how this gives z = 0 (since f is the nonzero coefficient of in fz = 0) but, I'm confused for the rest.

4. Feb 25, 2013

### Dick

Re: If certain entries of this matrix are all nonzero, show that the o

Put z=0 into the other equations. What do they become?

5. Feb 25, 2013

### s3a

Re: If certain entries of this matrix are all nonzero, show that the o

Oops. I now see that x = y = z = 0. :)

So, because x = y = z or x_1 = x_2 = x_3 (depending on the notation chosen) shows that the vector x = 0 and the problem is solved, right?

6. Feb 25, 2013

### Dick

Re: If certain entries of this matrix are all nonzero, show that the o

Yes.

7. Feb 25, 2013

### s3a

Re: If certain entries of this matrix are all nonzero, show that the o

Yay. :)

Thanks.