Matrix Inverse Question

Question

Let $C$ be an invertible 2x2 matrix such that:

$$C^{-1} \cdot \begin{bmatrix}1 \\ 2\end{bmatrix} = \begin{bmatrix}3 \\ 4\end{bmatrix}$$

$$C^{-2} \cdot \begin{bmatrix}9 \\ 5\end{bmatrix} = \begin{bmatrix}3 \\ 4\end{bmatrix}$$

Find $2\times2$ matrices $A$ and $B$ so that $CA=B$ and solve for $C$.

Answer 1

$$\pmatrix{1\cr2\cr}=C\pmatrix{3\cr4\cr}$$

$$\pmatrix{9\cr5\cr}=CC\pmatrix{3\cr4\cr}=C\pmatrix{1\cr2\cr}$$

Now do you see what to use for $A$ and $B$?

Answer 2

Hint $\rm\ C\,(C^{-1} u = v)\:\Rightarrow\: u\, =\, \color{#C00}{C v}$

And $\rm\ C^2\, (C^{-2}w = v)\:\Rightarrow\: w = C(\color{#C00}{Cv}) = Cu$

Thus $\rm\ C\, (u,v) = (w,u)$