Finding All $2\times 2$ Matrices That Satisfy $A^2=A$

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In summary, the matrices of order $2\times 2$ that satisfy $A^2 = \begin{pmatrix}1 & 1\\ 0 & 1\end{pmatrix}$ are $A = \begin{pmatrix}1 & \frac{1}{2}\\ 0 & 1\end{pmatrix}$ and $A = \begin{pmatrix}-1 & -\frac{1}{2}\\ 0 & -1\end{pmatrix}$.
  • #1
juantheron
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How can i find all matrix $A$ of order $2\times 2$ that satisfy the $A^2 = \begin{pmatrix}1 & 1\\
0 & 1
\end{pmatrix}$
 
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  • #2
jacks said:
How can i find all matrix $A$ of order $2\times 2$ that satisfy the $A^2 = \begin{pmatrix}1 & 1\\
0 & 1
\end{pmatrix}$

Hi jacks,

Let \(A = \begin{pmatrix}a & b\\ c & d\end{pmatrix}\). Then \(A^2 = \begin{pmatrix}a^2+bc & ab+bd\\ ac+cd & bc+d^2\end{pmatrix}\)

Since, \(A^2 = \begin{pmatrix}1 & 1\\ 0 & 1\end{pmatrix}\)

\[a^2+bc=1~~~~~~~(1)\]

\[ab+bd=1~~~~~~~(2)\]

\[bc+d^2=1~~~~~~~~(3)\]

\[ac+cd=0~~~~~~~~(4)\]

From (1) and (3), \(a=\pm d\). But (4) implies that, \(a=d\). Then substituting for \(d\) in (4), \(ac=0\). If \(a=0\Rightarrow d=0\) and we get a contradiction from (2). Therefore, \(c=0\). Then by (1) and (3), \(a=d=\pm 1\). Finally by (2), \(b=\pm\frac{1}{2}\). So we have two sets of answers,

\[a=d=1,~b=\frac{1}{2},~c=0\mbox{ or }a=d=-1,~b=-\frac{1}{2},~c=0\]
 

FAQ: Finding All $2\times 2$ Matrices That Satisfy $A^2=A$

What is the definition of a $2\times 2$ matrix?

A $2\times 2$ matrix is a rectangular array of numbers with 2 rows and 2 columns. It can be written in the form $\begin{bmatrix}a & b\\c & d\end{bmatrix}$ where $a, b, c,$ and $d$ are real numbers.

What does it mean for a matrix to satisfy $A^2 = A$?

When a matrix satisfies $A^2 = A$, it means that when the matrix is multiplied by itself, the resulting matrix is equal to the original matrix. In other words, the matrix is idempotent.

How many $2\times 2$ matrices satisfy $A^2 = A$?

There are an infinite number of $2\times 2$ matrices that satisfy $A^2 = A$. This is because any matrix with the values $a = 0, b = 0, c = 0,$ and $d = 0$ will satisfy the equation. Additionally, any matrix with the values $a = 1, b = 0, c = 0,$ and $d = 1$ will also satisfy the equation.

Can you provide an example of a $2\times 2$ matrix that satisfies $A^2 = A$?

One example of a $2\times 2$ matrix that satisfies $A^2 = A$ is $\begin{bmatrix}1 & 0\\0 & 0\end{bmatrix}$. When this matrix is multiplied by itself, the resulting matrix is $\begin{bmatrix}1 & 0\\0 & 0\end{bmatrix}$, which is equal to the original matrix.

How can I find all $2\times 2$ matrices that satisfy $A^2 = A$?

To find all $2\times 2$ matrices that satisfy $A^2 = A$, you can use algebraic methods such as setting up a system of equations and solving for the variables. Another method is to use geometric intuition and visualize the possible solutions on a graph. You can also use computer software or calculators to generate a list of matrices that satisfy the equation.

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