# [Matrices Inverses] Finding the inverse equality to I doesn't work

• June 1st 2010, 08:14 PM
Cthul
[Matrices Inverses] Finding the inverse equality to I doesn't work
• June 1st 2010, 08:47 PM
Soroban
Hello, Cthul!

What doesn't work?

Quote:

Find the inverse of: . $\begin{bmatrix}\text{-}2&\text{-}7 \\ 1&4\end{bmatrix}$

. . $(A)\;\begin{bmatrix}4&7\\ \text{-}1&\text{-}2\end{bmatrix}\qquad (B)\;\begin{bmatrix}\text{-}2&1\\7&\text{-}4\end{bmatrix} \qquad (C)\;\begin{bmatrix}\text{-}4&\text{-}7 \\ 1 & 2 \end{bmatrix}$ . . $(D)\;\begin{bmatrix}1 & \text{-}7 \\ 2&\text{-}4\end{bmatrix}\qquad (E)\;\begin{bmatrix}\text{-}1 & 2 \\ 4&\text{-}7\end{bmatrix}$

Answer (C) works . . . check it out!

• June 1st 2010, 09:01 PM
Cthul
It makes $\begin{bmatrix}1&0 \\ 0&1\end{bmatrix}$
I tried finding an inverse.
I got:
$\frac{1}{15}\begin{bmatrix}4&1 \\ \text{-}7&\text{-}2\end{bmatrix}$ $\begin{bmatrix}1&0 \\ 0&1\end{bmatrix}=A$
How'd you find it works for C?

EDIT: I see what I did wrong now, nevermind. I know how to do it now.