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## Find all solutions to the systems of equations by row-reducing (3)

Solution to Linear Algebra Hoffman & Kunze Chapter 1.3 Exercise 1.3.3

Solution: The system $AX=2X$ is
$$\left[\begin{array}{ccc}6&-4&0\\4&-2&0\\-1&0&3\end{array}\right]\left[\begin{array}{c}x\\y\\z\end{array}\right]=2\left[\begin{array}{c}x\\y\\z\end{array}\right]$$which is the same as
\begin{alignat*}{1}
6x-4y&=2x\\
4x-2y&=2y\\
-x+3z&=2z
\end{alignat*}which is equivalent to
\begin{alignat*}{1}
4x-4y&=0\\
4x-4y&=0\\
-x+z&=0
\end{alignat*}The matrix of coefficients is
$$\left[\begin{array}{ccc}4&-4&0\\4&-4&0\\-1&0&1\end{array}\right]$$which row-reduces to
$$\left[\begin{array}{ccc}1&0&-1\\0&1&-1\\0&0&0\end{array}\right]$$Thus the solutions are all elements of $F^3$ of the form $(x,x,x)$ where $x\in F$.

The system $AX=3X$ is
$$\left[\begin{array}{ccc}6&-4&0\\4&-2&0\\-1&0&3\end{array}\right]\left[\begin{array}{c}x\\y\\z\end{array}\right]=3\left[\begin{array}{c}x\\y\\z\end{array}\right]$$which is the same as
\begin{alignat*}{1}
6x-4y &=3x\\
4x-2y&=3y\\
-x+3z&=3z
\end{alignat*}
which is equivalent to
\begin{alignat*}{1}
3x-4y&=0\\
x-2y&=0\\
-x&=0
\end{alignat*}The matrix of coefficients is
$$\left[\begin{array}{ccc}3&-4&0\\1&-2&0\\-1&0&0\end{array}\right]$$which row-reduces to$$\left[\begin{array}{ccc}1&0&0\\0&1&0\\0&0&0\end{array}\right]$$Thus the solutions are all elements of $F^3$ of the form $(0,0,z)$ where $z\in F$.