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So, I want to find the pivot columns to the matrix A:

A=

-1 1 -5 0 2

3 0 6 1 -1

2 -1 7 2 1

2 -7 25 5 1

and after that I want to solve Ax = b where b is (I kind of want to solve Ax = 0 as well, but let's skip that...):

b=

a+3

2a

-a-1

-9a-5

My thought was to solve everything at the same time, and I started doing that by gausselimination. The problem is that I've been stuck for a while with this:

C=

15 0 -15 0 0

0 3 0 0 0

0 0 -6 -1 0

0 0 0 5 10

and

d=

26a+7

a+2

2a

2a+14

The zero at row 2, column 3 is giving me a headache, and I don't know how to solve it at all. Is it possible I made a mistake while doing these row operations?

I'm very thankful for any kind of advice!

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# Find pivot columns

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