Old 17th August 2012, 02:37   #1
swingdjted
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basic algebra question/riddle

I've reviewed this and can't seem to find the error. Where does this go wrong?

Can someone go from line 4 to line five by arbitrarily tossing in the -2ab to both sides? I know it seems equal when you do that, as it looks like you can toss anything in there as long as it's balanced, but I don't know for sure. Help me out here.


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Old 17th August 2012, 03:12   #2
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if a=b then on the 5th line where -2ab is introduced the expression on the left-hand side of the equation simplifies to zero and also does the right-hand side:
code:
0=0

Both expressions on either side of the equality gets canceled out to zero. By the looks of it, it's a deliberate attempt to obfuscate the effect of adding -2ab to both sides.
code:
2*Squared(a)-2ab=0 if a=b

http://en.wikipedia.org/wiki/Mathematical_fallacy

Then there's also that pesky thing in algebra that you can't divide by zero, which is what the last step does, dividing both sides by (a squared - a*b), which is equal to zero if a=b.
http://www.math.toronto.edu/mathnet/...fs/guess8.html

Last edited by thinktink; 17th August 2012 at 03:45. Reason: grammar
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Old 17th August 2012, 04:06   #3
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Quote:
Originally Posted by thinktink View Post
Then there's also that pesky thing in algebra that you can't divide by zero, which is what the last step does, dividing both sides by (a squared - a*b), which is equal to zero if a=b.
Oh fuck, wait. No, no, no. Bad things happens when you divide by zero.

Shit. You can't do that!

There are hundreds of variants of this and they all have the same flaw: division by zero somewhere.
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Old 17th August 2012, 04:17   #4
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The first thing you mentioned is what I missed. I failed to simplify on my own rather than look at the picture. Had I done that after each presented line, the rest of what you typed would have been blatantly obvious.

Thank you for that, because I was turning my head a little sideways when looking at that. I'm better now.

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Old 17th August 2012, 04:53   #5
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You're welcome. Debunking fallacious algebra proofs since 2012.
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Old 17th August 2012, 15:02   #6
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a squared is not necessarily equal to 2*a

that's where the formula falls apart.

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Old 17th August 2012, 15:55   #7
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It didn't have that error, because the 2a squared came from the line above where it was a squared plus a squared.

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Old 18th August 2012, 02:51   #8
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I swear, the forums could benefit with superscript and subscript formatting tags.
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Old 18th August 2012, 03:03   #9
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Some people type "A squared" as a^2. The same people often type "A sub 2" as av2 although the former is more common than the latter.

Also you could type the tag out even though it doesn't do anything e.g. "A squared" becomes A[superscript]2[/superscript] or "A sub 2" becomes A[subscript]2[/subscript], but that would just look silly.

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Old 18th August 2012, 19:42   #10
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I've seen other vbb forums use [sup]2[/sup] and [sub]2[/sub], why it's not here I don't know.

if all you need are superscript numbers, then unicode is your friend:
¹²³⁴⁵⁶⁷⁸⁹⁰ⁱ⁽⁾⁺⁻⁼ⁿ just copy and paste what you need
PHP Code:
       a=b
      a²
=ab
   a²
+=+ab
     2a²
=+ab
 2a²
-2ab=+ab-2ab
 2a²
-2ab=-ab
2
(-ab)=1(-ab)
       
2=


oh, and subscript:
ᵢᵣᵤᵥᵦᵧᵨᵩᵪ₀₁₂₃₄₅₆₇₈₉₊₋₌₍₎ₐₑₒₓₔ
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Old 18th August 2012, 20:41   #11
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Ahh, nice. I'll have to keep this thread in mind for copy pasta from your post. I used to use unicode a lot because I had a nice small document with a lot of the most common unicode characters there for copy-paste, but I've since lost the document.

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Old 18th August 2012, 20:56   #12
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If you know what you are looking for, you can search the Character Map in Windows
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Old 8th October 2012, 03:12   #13
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This did my head in for a few minutes, until I realised that 1(X) = n(X), so long as X = 0, which is what this riddle does.

Tricky bastards.

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