What is the Flaw in the 'Proof' That -1 Equals 1?

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Discussion Overview

The discussion centers around a purported proof that claims -1 equals 1, exploring the validity of the mathematical steps involved in the proof. Participants analyze the use of square roots and the properties of complex numbers, focusing on the implications of these operations.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the validity of the transition between steps in the proof, specifically the use of square roots with negative values, suggesting that the property of square roots does not apply here.
  • Another participant emphasizes the importance of considering both positive and negative square roots when dealing with complex numbers, arguing that the proof fails because it neglects to account for the signs of the square roots.
  • A participant reflects on the circular reasoning involved in determining which square root to take, using an example with real numbers to illustrate the potential for error in the proof.
  • A later reply clarifies that the original poster was not advocating for the proof but was seeking to understand the mistake within it.

Areas of Agreement / Disagreement

Participants generally agree that there is a flaw in the proof, particularly regarding the handling of square roots, but they present differing perspectives on the nature of the error and the implications of complex numbers.

Contextual Notes

Limitations include the assumption that the square root function behaves consistently across all domains, particularly with negative inputs, and the unresolved nature of how to appropriately handle signs in complex square roots.

spec138
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Hi all. I found this "proof" and was just wondering if there is an error in it or not, because I couldn't find it. Any ideas?

-1/1=1/-1
sqrt(-1/1)=sqrt(1/-1)
sqrt(-1)/sqrt(1)=sqrt(1)/sqrt(-1)
i/1=1/i
i*i/1=i*1/i
i^2/1=i/i
-1=1 ?
 
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sqrt(-1/1)=sqrt(1/-1)
sqrt(-1)/sqrt(1)=sqrt(1)/sqrt(-1)

The transition between these two steps is invalid. There is no reason you would be allowed to do this.

There is a basic property of the square root function that sqrt(a)*sqrt(b)=sqrt(a*b) where a and b are positive, but -1/1 isn't positive so we don't get to use that here.

I've actually seen this one before a page promoting "Time Cube" theory!
 
The problem comes from writing simple "sqrt" functions rather than "+/-sqrt". IOW, whenever you take the square root, you have to take into consideration that there are two square roots of any complex number, which differ by a factor of -1. Deciding which roots to take to maintain an equality often requires working through exactly the kind of computation you have shown.

I would have looked at the second line in your "proof" and decide which sign each sqrt should have. To do that I would go through pretty much the proof you have, and when I got "-1=1", I would say, "Oh - that's not it - I guess the two sqrts have to have opposite signs to maintain the equality."

I know that looks circular, but it's really how you decide which root to take. It might be clearer with pure real numbers:

(-2)^2 = (2)^2
sqrt((-2)^2) = sqrt((2)^2)
-2 = 2

oops ... should have had a minus sign in line 2!
 
Just to add this wasn't my proof and I knew it wasn't true. I just couldn't find the mistake. Thanks guys.
 

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