Can you spot the mistake in this equation?

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In summary, the conversation revolves around finding the mistake in the equation 1=\sqrt{1}=\sqrt{(-1)(-1)}=\sqrt{(-1)^2}=-1. The mistake is that \sqrt{(-1)^2} should not equal -1, but instead should be equal to 1. The discussion also brings up the definition of \sqrt{x} as a real valued function and the fact that it can only have one value for each x. The conversation ends with the realization that this argument has been brought up multiple times before.
  • #1
mathlover1
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Somebody find the mistake here:

[tex]
1=\sqrt{1}=\sqrt{(-1)(-1)}=\sqrt{(-1)^2}=-1 \Rightarrow 1=-1
[/tex]​
 
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  • #2
Square root of 1 is not 1, it is either 1 or -1.
 
  • #3
Since when is the (square root of -1)-squared equal to -1?

-1 squared is 1.
The root of 1 is 1, not -1.
 
  • #4
DaveC426913 said:
The root of 1 is 1, not -1.

yes it's -1 because (-1)^2=1 from the definition ;)
 
  • #5
mathlover1 said:
yes it's -1 because (-1)^2=1 from the definition ;)

So do you or do you not see the flaw?
 
  • #6
The error is here
[tex]
\sqrt{(-1)^2} \neq -1
[/tex]

[tex]\sqrt{(-1)^2} = |-1|=1[/tex]
 
  • #7
Well-done Njama, your answer is the correct one.
 
  • #8
mathlover1 said:
Well-done Njama, your answer is the correct one.

Which is precisely what I said in post 3.
 
  • #9
Borek said:
Square root of 1 is not 1, it is either 1 or -1.
Not true.
mathlover1 said:
yes it's -1 because (-1)^2=1 from the definition ;)
Not true.
njama said:
The error is here
[tex]
\sqrt{(-1)^2} \neq -1
[/tex]

[tex]\sqrt{(-1)^2} = |-1|=1[/tex]
True! [tex]\sqrt{x}[/itex], as a real valued function, must have only one value for each x and it is defined as "the positive number y such that [itex]y^2= x[/itex]"
mathlover1 said:
Well-done Njama, your answer is the correct one.
Then why did you deny it in your post quoted above?
 
  • #10
Is it just me, or is this the 3rd time in the last month somebody has posted the very same 1=-1 argument?
 
  • #11
Well, it really belongs in the Riddles section.
 

Related to Can you spot the mistake in this equation?

1. How can 1 be equal to -1?

This may seem counterintuitive, but it is possible for 1 to equal -1 in certain mathematical equations. In this case, the statement "1=-1" is a false statement, as it goes against the basic principles of arithmetic.

2. What is the mistake in the equation 1=-1?

The mistake in this equation is that the equals sign is being used incorrectly. The equals sign is typically used to show that two quantities are equal, but in this case, it is being used to assign a value. This is incorrect, as 1 and -1 are not equal values.

3. Can this equation be solved?

No, this equation cannot be solved as it is a false statement. There is no solution that can make 1 equal to -1.

4. Is there any scenario where 1=-1 is true?

No, there is no scenario in mathematics where 1 can equal -1. In order for an equation to be true, it must follow the rules of mathematics, and this equation does not.

5. How can we avoid making this mistake in the future?

To avoid making this mistake, it is important to have a clear understanding of basic arithmetic principles. Remember that the equals sign should be used to show that two quantities are equal, not to assign a value. Double-checking equations and calculations can also help catch any mistakes before they are made.

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