Answer to Question #176506 in Linear Algebra for Azuzile

Question #176506

Consider the following system of linear equations:

x − y + z = 1

x + y − 2z = 2

2x − z = 3

(a) How many solutions does the system have? Justify your

answer


1
Expert's answer
2021-03-31T17:01:14-0400

Let us find the rank of the extended matrix of the system of equations:

(111112201123)I2I(111023023111)II(111023000110)\left( {\left. {\begin{matrix} 1&{ - 1}&1\\ 1&1&{ - 2}\\ 2&0&{ - 1} \end{matrix}} \right|\begin{matrix} 1\\ 2\\ 3 \end{matrix}} \right)\begin{matrix} {}\\ { - I}\\ { - 2I} \end{matrix} \sim \left( {\left. {\begin{matrix} 1&{ - 1}&1\\ 0&2&{ - 3}\\ 0&2&{ - 3} \end{matrix}} \right|\begin{matrix} 1\\ 1\\ 1 \end{matrix}} \right)\begin{matrix} {}\\ { - II} \end{matrix} \sim \left( {\left. {\begin{matrix} 1&{ - 1}&1\\ 0&2&{ - 3}\\ 0&0&0 \end{matrix}} \right|\begin{matrix} 1\\ 1\\ 0 \end{matrix}} \right)

Since the ranks of the main and extended matrix are equal to each other, but not equal to the number of equations:

rank(A)=rank(AB)=23rank(A) = rank(A|B) = 2 \ne 3 , the system of equations is consistent, but not defined, and therefore has an infinite number of solutions


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