Solving Gaussian Elimination Homework

[XodoX]

Homework Statement

http://img189.imageshack.us/i/50468910.png/

Homework Equations

The Attempt at a Solution

I'm wondering how I do this? I don't really know. Supposed to be 17 rows. How many columns and how many each row? No clue! And how do I get the numbers for the matrix here?

 

[tiny-tim]

Hi XodoX!

I'm wondering how I do this? I don't really know. Supposed to be 17 rows. How many columns and how many each row? No clue! And how do I get the numbers for the matrix here?

The question says "the matrix A and b to have 17 rows" …

well, if Ab makes sense, that means A must have 17 columns, doesn't it?

(And there's 17 equations given so it's one row for each equation.)

 

[XodoX]

Oh, ok. I thought one row for each joint. So, the first row would be -1, +1, +1 , which would be A, and 0 would be b. Is that correct?

Also, line # 4 says : F=f3-10=0 I suppose I have to move the 10 here, because all the other rows don't have numbers in them.

 

[tiny-tim]

Hi XodoX!

Oh, ok. I thought one row for each joint. So, the first row would be -1, +1, +1 , which would be A, and 0 would be b. Is that correct?

Also, line # 4 says : F=f3-10=0 I suppose I have to move the 10 here, because all the other rows don't have numbers in them.

No, your equation is Ab = c, where b is (f₁,…f₁₇), and c contains all the 0s and 10s and 15.

So the first row is … ? ​

 

[XodoX]

Hi XodoX! No, your equation is Ab = c, where b is (f₁,…f₁₇), and c contains all the 0s and 10s and 15.

So the first row is … ? ​

Sorry, I don't get it. I thought each equation is one row.. F1, F4, and F5 in the first row. If that's not it, I don't get it then. -F1 + F4 + F5 =0

 

[tiny-tim]

Sorry, I don't get it. I thought each equation is one row.. F1, F4, and F5 in the first row.

Yes, but there's 17 numbers in the first row (and every row), not just 3.

 

[XodoX]

Yes, but there's 17 numbers in the first row (and every row), not just 3.

But it says it has 17 rows, so I figured it must be the F values and numbers of each line for each row.

1. row -F1 + F4 + F5=0

2. row -F1 - F3 - F5 = 0

etc.

 

[tiny-tim]

Hi XodoX!

(just got up :zzz: …)

No, the matrix A (and the vector c) only has numbers, not Fs … the Fs are only in the vector b.

For example, the first row of A is (-1,0,0,1,1,0,0,0,0,0,0,0,0,0,0,0,0).

(and so -F₁ + F₄ + F₅ is the first row of the vector Ab, which has to be equal to the first row of the vector c, which of course is 0)

 

[XodoX]

Hi XodoX!

(just got up :zzz: …)

No, the matrix A (and the vector c) only has numbers, not Fs … the Fs are only in the vector b.

For example, the first row of A is (-1,0,0,1,1,0,0,0,0,0,0,0,0,0,0,0,0).

(and so -F₁ + F₄ + F₅ is the first row of the vector Ab, which has to be equal to the first row of the vector c, which of course is 0)

Hi.

I know it dosen't have F's. Just did it because I thought it would better illustrate what I meant. I honestly don't know where you got the 17 numbers from. From what I see, it should be 17 rows down, and each rown has about 3-4 numbers. I understand that row 1 is -F₁ + F₄ + F₅ and then the 0. But that's just 4 numbers and not 17 for this particular row. Sorry, I don't know why this seems so difficult. It's actually not.

 

[tiny-tim]

Each row of A, when multiplied by b, gives you the LHS of one of the equations.

For example, the row I gave you, multiplied by b = (F₁, … F₁₇), gives you the LHS of equation #1 …

I don't understand why you don't see that. ​

 

[XodoX]

What's there to multiply b ? b is 0 there. It's already on the right side of the equation. Why would I even multiply by b?

 

[tiny-tim]

I don't think we're talking about the same thing

I'm talking about an equation Ab = c,

where A is a matrix with numbers (mostly 0), b is the vector (F₁, …F₁₇), and c is a vector with 0s 10s and 15s