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  #1  
Old 08-22-2009, 05:00 PM
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what just happened here?(wanring: contains math)

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Given:a=b
a2=ab
a2-b2=ab-b2
(a+b)(a-b)=b(a-b)
(a+b)=b
(a+a)=a
2a=a
2=1
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  #2  
Old 08-22-2009, 05:10 PM
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Aren't there rules against variables having the same value? If not, there certainly should be.
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Old 08-22-2009, 05:24 PM
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Quote:
Originally Posted by Unrepresented View Post
Aren't there rules against variables having the same value? If not, there certainly should be.
Well, techinically, it's if a=b, it doesn't matter what you do. if 1=1, then all the rest is just conjecture and is incorrect.

math is weird.
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Old 08-22-2009, 05:28 PM
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(a+b)(a-b)=b(a-b)
(a+b)=b

Didn't you divide by zero there (since a-b = 0)?
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Old 08-22-2009, 05:29 PM
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Quote:
Originally Posted by drteeth View Post
(a+b)(a-b)=b(a-b)
(a+b)=b

Didn't you divide by zero there (since a-b = 1-1)?
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Old 08-22-2009, 05:29 PM
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  #7  
Old 08-22-2009, 05:31 PM
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Quote:
Originally Posted by drteeth View Post
(a+b)(a-b)=b(a-b)
(a+b)=b

Didn't you divide by zero there (since a-b = 0)?

He just imploded the universe!
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  #8  
Old 08-22-2009, 05:39 PM
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Originally Posted by Thunderscreech View Post
That was my point. Having two variables with the same value creates a situation where such circumstances are potential.

Both variables shouldn't be given different the presumption of different values if they're not different.
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Old 08-22-2009, 06:23 PM
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Quote:
Originally Posted by drteeth View Post
(a+b)(a-b)=b(a-b)
(a+b)=b

Didn't you divide by zero there (since a-b = 0)?
^ Correct
  #10  
Old 08-22-2009, 09:32 PM
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Yup, that's the classic hidden zero.
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  #11  
Old 08-22-2009, 09:36 PM
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That was my point. Having two variables with the same value creates a situation where such circumstances are potential.

Both variables shouldn't be given different the presumption of different values if they're not different.
I don't think 2 variables being equal is a problem. Just do it again by replacing every "b" by "a" and you will come to the same exact conclusion.

Whenever you see those weird mathematical solvings (usually it ends up with 1+1=3 or something like that), there's always an obvious mistake to anyone used to maths.

To spot them quickly, it is almost always a division by 0 or, in some cases, a bad solving of a polynome (ex : a^2+2b+c) when there is 2 theoretical answers but only one is part of real numbers (and the wrong one is displayed on the next line ...). These are the 2 tricks to spot those quickly.
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  #12  
Old 08-23-2009, 01:47 AM
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Or a rounding error:

1 = 3/3
1 = 0.3333.... * 3
1 = 0.99999....

the .333 and .999 are recurring

i know a good one.. ill have to try and remember it.
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  #13  
Old 08-23-2009, 06:50 AM
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Technically, isn't .999 recurring the same thing as 1 though?

-Dash
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  #14  
Old 08-23-2009, 07:26 AM
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Quote:
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Technically, isn't .999 recurring the same thing as 1 though?

-Dash
Yes.
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  #15  
Old 08-23-2009, 07:54 AM
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Originally Posted by Unrepresented View Post
That was my point. Having two variables with the same value creates a situation where such circumstances are potential.

Both variables shouldn't be given different the presumption of different values if they're not different.
Well, that's the whole point of variables... they're variable.

Meaning they can be any value. In the off chance that 2 different variables are the same value the universe shouldn't implode.


a2-b2=ab-b2 , just proves 0 = 0 when you have nothing you shouldn't go on.
But you do and you divide by zero.



(a+b)=b
(a+a)=a

a=a-a
a=0
therefore b=0

???

I'm searching if a and b can be another value

Last edited by René_Julien : 08-23-2009 at 07:59 AM.
  #16  
Old 08-23-2009, 08:01 AM
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No, because as along as a=b, you can't divide by (a-b), since it means it is 0.
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  #17  
Old 08-23-2009, 08:09 AM
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Quote:
Originally Posted by L-A View Post
No, because as along as a=b, you can't divide by (a-b), since it means it is 0.
Yup, I've been thinking about it again.
Since these comparisons started with the fact that a=b you have to keep respecting that.
Going from line 3 to 4 is dividing by zero.



Doesn't mean that in a formula some variables can't be the same value.
But this is not a formula... it's just a comparison worked out wrong.




BTW, is it allowed to divide zero by zero?

I know it's not allowed.
  #18  
Old 08-23-2009, 10:00 AM
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i'm starting school tomorrow, i'm taking this to my math Prof..
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  #19  
Old 08-23-2009, 11:04 AM
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Quote:
Originally Posted by René_Julien View Post
it's just a comparison worked out wrong.



I know it's not allowed.
sounds like this is it..
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