$\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 :)

Printable View

- Jun 9th 2010, 08:49 AMZaphFunny 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 AMundefined
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 AMZaph
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 AMundefined
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.

You're welcome! - Jun 9th 2010, 08:46 PMBacteriusQuote:

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.

*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 PMundefined
- Jun 9th 2010, 09:02 PMBacterius