Question

Solving a System of Equations in Three Variables

Find the complete solution of the linear system, or show that it is inconsistent.

$\left\{\begin{array}{l}x-y-z=4\\ 2y+z=-1\\ -x+y-2z=5\end{array}\right.$

Short answer

The solution for the given set of linear equations in three variables is $(x,y,z)=(2,1,-3)$

Step-by-step solution

Step 1. Given information

We are given the set of linear equations that is-

$\left\{\begin{array}{l}x-y-z=4\\ 2y+z=-1\\ -x+y-2z=5\end{array}\right.$

Step 2. Concept used

We need to find the complete solution of the given system of linear equations and show if it is inconsistent

We will eliminate variables using the gauss elimination method.

Then we will solve using back substitution and check the consistency of the system.

Step 3. Calculation

Given the system of linear equations is

$\left\{\begin{array}{l}x-y-z=4\\ 2y+z=-1\\ -x+y-2z=5\end{array}\right.$

We will replace the third equation by adding the first equation to it which is

$\begin{array}{l}(x-x)+(-y+y)+(-z-2z)=4+5\\ -3z=9\\ z=-3\end{array}$

Then, we can write the system of linear equations as

$\left\{\begin{array}{l}x-y-z=4\\ 2y+z=-1\\ z=-3\end{array}\right.$

Now, we will substitute this above result in the second equation which gives

$\begin{array}{l}2y+z=-1\\ 2y-3=-1\\ 2y=2\\ y=1\end{array}$

We will substitute the above values of y and z in the first equation which gives

localid="1651560840344" $$\begin{gathered} x-y-z=4 \\ x-1-(-3)=4 \\ x-1+3=4 \\ x+2=4 \\ x=2 \end{gathered}$$