# Funny little thing i stumpled upon

• Jun 9th 2010, 08:49 AM
Zaph
Funny little thing i stumpled upon
\$\displaystyle a=b => a^2=ab =>a^2-b^2=ab-b^2=>\$\$\displaystyle (a+b)(a-b)=b(a-b) => a+b=b =>2b=b => 2=1\$

It's pretty simple to find the 'error', but a funny equation to show students :)
• Jun 9th 2010, 08:57 AM
undefined
Quote:

Originally Posted by Zaph
\$\displaystyle a=b => a^2=ab =>a^2-b^2=ab-b^2=>\$\$\displaystyle (a+b)(a-b)=b(a-b) => a+b=b =>2b=b => 2=1\$

It's pretty simple to find the 'error', but a funny equation to show students :)

I remember it being funny at first but got old pretty quickly. There's some discussion on this thread. I post this mainly to prevent a repeat thread.

By the way, you can get nicer implication arrows using \Rightarrow \$\displaystyle \Rightarrow\$
• Jun 9th 2010, 09:02 AM
Zaph
Ow, sorry for repost then (Itwasntme) , im fairly new to this forum, so.. Thanx for the TeX help though(Clapping)
• Jun 9th 2010, 09:16 AM
undefined
Quote:

Originally Posted by Zaph
Ow, sorry for repost then (Itwasntme) , im fairly new to this forum, so..

Not sure what the "Ow" was meant to represent.. some comments in the other thread were a bit harsh, and I certainly didn't mean to make you feel like you'd stepped out of line or posted something dumb. It's good to realise though that to people experienced in mathematics, the errors in these types of "proofs" tend to stand out immediately and appear rather silly.

Quote:

Originally Posted by Zaph
Thanx for the TeX help though(Clapping)

You're welcome!
• Jun 9th 2010, 08:46 PM
Bacterius
Quote:

It's good to realise though that to people experienced in mathematics, the errors in these types of "proofs" tend to stand out immediately and appear rather silly.
Well it can be shown that if you come up with a result that contradicts the original axioms used, then either somehow you made the axioms fail in your line of thinking, either you failed in your line of thinking. The first case hasn't been stumbled upon yet ... and I wonder if it can be proved that they can't fail *accidentally* (Wondering)

But yes, it's quite fun sometimes to check out those "failproofs". You can check out this link, there is a more subtile version of the same contradiction, which is quite harder to see and a lot more interesting IHMO.

Googling "Fallacious proof" also returns a lot of interesting results :)
• Jun 9th 2010, 08:59 PM
undefined
Quote:

Originally Posted by Bacterius
The first case hasn't been stumbled upon yet ... and I wonder if it can be proved that they can't fail *accidentally* (Wondering)

See Gödel's second incompleteness theorem.
• Jun 9th 2010, 09:02 PM
Bacterius
Quote:

Originally Posted by undefined

Oh, what ? How could I forget Gödel's ? I need to remember my mathematics ... Thanks though.