Can a Direct Proof Show That 1-A is its Own Inverse?

In summary, my teacher said that when proving something, I can't start out with what I'm trying to prove. But what if it is an "if this than that proof" For example, if A(squared)=A, then I-A=(I-A)inverse.
  • #1
torquerotates
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So my teacher said that when proving something, I can't start out with what I'm trying to prove. But what if it is an "if this than that proof"

For example,

If A(squared)=A, then I-A=(I-A)inverse

Well, I started using what I'm trying to prove by multiplying both sides by I-A

I get (I-A)squared=I
implies I-4A+4AA=I
implies I-4A+4A=I b/c A(squared)=A using the hypothesis
implies I+0=I
implies I=I both sides equal

The thing is that I have proven that if AA=A, then I-A=(I-A)inverse by using the hypothesis somewhere in the solution. Would this be a logical conclusion?
 
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  • #2
No, everything that you have just written is complete nonsense. First of all, (I-A)^2 = I - 2A + A^2, so your proof that I=I is flawed anyway.

What you have tried to do is to take some statement and to use it to derive a true statement. However, I can easily derive a true statement from a false one. For instance, you have assumed that (I-A)^2 = I-4A+4A^2 and that A^2 = A. These two statements are not in general true (This is in fact only true if A = 0). Then you have used these statements to derive that I=I, which is true, but meaningless.
 
  • #3
if every step in your proof is correct and reversable, then after geting to a true stTEMENT, just reverse field and reason bCKWrds to the desired statement.
 
  • #4
here i would try proving the statement directly, i.e. ask whether indeed 1-A is its own inverse by squaring it and seeing if you get 1. along the way you get to replace A^2 by A.

i.e. (1-A)^2 = 1-2A + A^2 = 1-2A+A = 1-A. this does not seem to prove what you asked for.

this seems to prove that if you start from a projection operator and subtract it from 1, you get another projection operator.
 

FAQ: Can a Direct Proof Show That 1-A is its Own Inverse?

What is logic?

Logic is the study of reasoning and the principles that govern valid and coherent thinking.

Why is it important to understand logic?

Understanding logic is important because it allows us to think critically and make sound judgments based on evidence and reasoning.

What is the difference between deductive and inductive reasoning?

Deductive reasoning is a type of logical reasoning where conclusions are drawn from general principles or premises. Inductive reasoning involves drawing conclusions based on specific observations or evidence.

How can I improve my logical thinking skills?

One way to improve your logical thinking skills is to practice solving puzzles or problems that require logical reasoning. You can also read books or take courses on logic and critical thinking.

Can logical thinking be applied in everyday life?

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