Problem: Consider the following basis of $\mathbb{R} ^2$
\[
\begin{bmatrix}
1 \\
2 \\
\end{bmatrix},
\begin{bmatrix}
3 \\
5 \\
\end{bmatrix}.
\]
Find the coordinates for the vector $\begin{bmatrix}
-2 \\
4 \\
\end{bmatrix}$ in therms of the basis $B$. Also, if the linear transformation $T$ is given by
\[
T : \mathbb{R} ^2 \to \mathbb{R} ^2, (x,y) \mapsto (2x - y, 3x - 2y),
\]
find the matrix representation of $T$ with respect to the basis $B$.
Solution: Let
\[
v_1 = \begin{bmatrix}
1 \\
2 \\
\end{bmatrix}, \text{ and } v_2 = \begin{bmatrix}
3 \\
5 \\
\end{bmatrix}.
\]
We need to find $\alpha ,\beta \in \mathbb{R}$ such that
\[
\alpha \begin{bmatrix}
1 \\
2 \\
\end{bmatrix}+ \beta \begin{bmatrix}
3 \\
5 \\
\end{bmatrix} = \begin{bmatrix}
-2 \\
4 \\
\end{bmatrix}.
\]
Equivalently, we want to solve the system of linear equations
\begin{align*}
\alpha + 3\beta & = -2 \\
2\alpha + 5\beta & = 4.
\end{align*}
Solving the above two equations, give $\alpha = 22$ and $\beta =-8$. Therefore, the coordinates in terms of the basis $B$ are
\[
\begin{bmatrix}
22 \\
-8 \\
\end{bmatrix}.
\]
At first, we will find the image of the basis vectors under the linear transformation $T$.
\begin{align*}
w_1 = T \begin{bmatrix}
1 \\
2 \\
\end{bmatrix} = \begin{bmatrix}
0 \\
-1 \\
\end{bmatrix}, \text{ and }
w_2 = T \begin{bmatrix}
3 \\
5 \\
\end{bmatrix} = \begin{bmatrix}
1 \\
-1 \\
\end{bmatrix}.
\end{align*}
One way to find the matrix of the linear transformation is to write the vectors $w_1$ and $w_2$ in terms of $v_1$ and $v_2$. The matrix of $T$ will be given by the coordinates for the vectors $w_1$ and $w_2$. Another way is to apply the change of the basis matrix. It is easy to find the matrix with respect to the standard basis, say $T_s$, and then the matrix with respect to $B$ will be
\[
T_B = \begin{bmatrix}
T_s ^{-1} \left( w_1 \right) & T_s ^{-1} \left( w_2 \right) \\
\end{bmatrix}.
\]
Here we have
\[
T_s ^{-1} = \begin{bmatrix}
1 & 3 \\
2 & 5 \\
\end{bmatrix}^{-1} = \begin{bmatrix}
-5 & 3 \\
2 & -1 \\
\end{bmatrix}.
\]
Therefore,
\begin{align*}
T_B & = \begin{bmatrix}
T_s ^{-1} \left( w_1 \right) & T_s ^{-1} \left( w_2 \right) \\
\end{bmatrix} \\
& = \begin{bmatrix}
-3 & -8 \\
1 & 3 \\
\end{bmatrix}.
\end{align*}