Exercise 2.2.4. Consider the system of linear equations represented by the following matrix:

Find the echelon matrix , a set of basic variables, a set of free variables, and the general solution to . Then use elimination to find when the system has a solution, and express that solution as the sum of a particular solution and the general solution to . Finally, find the rank of .

Answer: We perform elimination on by subtracting 2 times the first row from the second row (i.e., using the multiplier ):

This completes elimination, and leaves us with the echelon matrix

and the factorization :

We now solve for

which is equivalent to

The pivot in is in column 2, so the (only) basic variable is and the free variables are , and .

From the first row of we have and thus . We then have

In considering the inhomogeneous system or

the elimination sequence above would produce the system or

From the second equation we must have or . Thus for to have a solution the vector must lie on the line passing through the origin and the point so that .

Taking produces the system

which after elimination becomes

The first equation gives or . Setting the free variables , , and all to zero produces the particular solution to the system

We can combine the particular solution to this system with the solution to to produce the general solution for the system

We can check this solution as follows:

The rank of is 1, the number of basic variables (or pivots).

UPDATE: I expanded the answer to conform to the presentation in the answer to exercise 2.2.5.

UPDATE 2: I corrected the value for used in various places. Thanks go to Zoi for catching this error!

NOTE: This continues a series of posts containing worked out exercises from the (out of print) book Linear Algebra and Its Applications, Third Edition by Gilbert Strang.

If you find these posts useful I encourage you to also check out the more current Linear Algebra and Its Applications, Fourth Edition, Dr Strang’s introductory textbook Introduction to Linear Algebra, Fourth Edition and the accompanying free online course, and Dr Strang’s other books.

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Hi! You have a mistake – some times you add an extra 1 in U.

Thanks for all your work!

Thank you for finding this error; I have updated the post to correct it. And thanks for reading the blog! I’m glad you find it useful.

I came across this blog post because when checking my solution in the back of the Third Edition, there were discrepancies. Thank you so much for this post. I was afraid I had the concepts wrong.

Thank you for commenting! I’m glad this post helped you resolve the discrepancies you had in your solution.