How Many 2x2 Matrices Equal I?

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Discussion Overview

The discussion revolves around the question of how many distinct 2x2 matrices exist such that the square of the matrix equals the identity matrix (A^2 = I). The scope includes mathematical reasoning and problem-solving related to linear algebra.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant asserts that there are 4 matrices that satisfy A^2 = I, but expresses uncertainty after being marked incorrect by a grader.
  • Another participant requests clarification on the proof attempt made by the first participant.
  • A participant provides an example of a matrix and suggests that there are many matrices that could satisfy the condition, encouraging the first participant to show their logic for further assistance.
  • One participant presents a system of four equations derived from the matrix entries and seeks guidance on how to solve them.
  • Another participant notes that two of the equations share a common term and suggests that setting certain variables to zero may lead to the four solutions, questioning the implications of the equation x + w = 0.

Areas of Agreement / Disagreement

Participants express differing views on the number of matrices that satisfy the condition A^2 = I, with no consensus reached on the correct answer or the reasoning behind it.

Contextual Notes

Participants have not resolved the mathematical steps involved in solving the equations, and there are dependencies on the assumptions made regarding the variables in the equations.

Tachyonie
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Assuming A is a 2x2 matrix how many different matricies exist such that A^2=I ?
I am 99% sure the answer is 4 but after putting that down as an answer with supporting evidence I was marked wrong (or atleast not fully correct) so I am stumped as to where to jump and whether or not the grader may ahve just messed up.
 
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Well, what was your attempt at proof?
 
Tachyonie said:
Assuming A is a 2x2 matrix how many different matricies exist such that A^2=I ?
I am 99% sure the answer is 4 but after putting that down as an answer with supporting evidence I was marked wrong (or atleast not fully correct) so I am stumped as to where to jump and whether or not the grader may ahve just messed up.
The grader gave you partial credit because you got the wrong answer but showed the supporting evidence that led you down the wrong path. Consider this matrix:

A = \bmatrix 0.6 & \phantom{-}1.6 \\ 0.4 & -0.6\endbmatrixThere are many, many more of such. Show your logic so we can help show where you went wrong.
 
I got 4 equations,

x^2 +yz=1
xy+yw=0
zx+wz=0
zy+w^2=1.

How do I solve this now?
 
letmeknow said:
I got 4 equations,

x^2 +yz=1
xy+yw=0
zx+wz=0
zy+w^2=1.

How do I solve this now?
Both of the middle equations (the ones equal to zero) have a common term. For example, xy+yw=0 is the same as (x+w)*y = 0. This means that at least one of x+w or y must be equal to zero. I suspect your four solutions result from setting y and z to zero. What if x+w=0?
 

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