# Proving inverse of a 2 x 2 matrix is really an inverse

• ChiralSuperfields
In summary, the conversation discusses a formula for solving linear equation systems in 2 variables using matrix operations. The formula involves swapping the diagonal entries, putting a minus sign in front of the entries on the side diagonal, and dividing by the determinant. This method is helpful for those who struggle with visualizing solutions and can be used for more complicated examples.
ChiralSuperfields
Homework Statement
Relevant Equations
For this,

Dose someone please know how ##ad - bc## and ##-cb + da## are equal to 1?

Many thanks!

They are not equal to 1. They are both divided by ad-bc, which gives 1.

PhDeezNutz and ChiralSuperfields
FactChecker said:
They are not equal to 1. They are both divided by ad-bc, which gives 1.
Oh thank you @FactChecker ! I see now

This formula
$$\begin{pmatrix}a&b\\c&d\end{pmatrix}^{-1}=\begin{pmatrix}d&-b\\-c&a\end{pmatrix} \cdot (ad-bc)^{-1}$$

is so easy to memorize ...
1. swap the diagonal entries ##a \leftrightarrow d##
2. put a minus sign in front of the entries on the side diagonal ##b \rightarrow -b\, , \,c\rightarrow -c##
3. and finally, divide by the determinant ##ad-bc.##
... that I use it whenever I have to solve a linear equation system in ##2## variables.

E.g. I wanted to write ##16 \cdot 24 ## and ##8\cdot 48## as ##(n-m)\cdot (n+m)## today. That goes:
\begin{align*}
\begin{pmatrix}1&-1\\1&1 \end{pmatrix}\cdot \begin{pmatrix}n\\m\end{pmatrix}=\begin{pmatrix}16\\24 \end{pmatrix}
\end{align*}
Then by doing the procedure as described I get
\begin{align*}
\begin{pmatrix}n\\m \end{pmatrix}=\begin{pmatrix}1&1\\-1&1 \end{pmatrix}\cdot \begin{pmatrix}16\\24 \end{pmatrix}\cdot \underbrace{(1\cdot 1- (-1\cdot 1))^{-1}}_{=1/2}=\begin{pmatrix}1&1\\-1&1 \end{pmatrix}\cdot \begin{pmatrix}8\\12 \end{pmatrix}=\begin{pmatrix}8+12\\ -8+12\end{pmatrix}=\begin{pmatrix}20\\4 \end{pmatrix}
\end{align*}
Same with the other product ##8\cdot 48.##

I know this was an easy example and could probably just "be seen". But I'm better with algorithms than "seeing" things. Once you get used to that procedure it is really easy to follow. Especially if the examples are a bit more complicated.

ChiralSuperfields

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