Found it!

[robinbloke]

I used to know this proof, but forgot it (as I do most things) but due to the wonders of the internet, I've found it again.


step 1: a = b

step 2: a² = ab [ after you multiply both sides by a ]

step 3: a² - b² = a² - b² [ subtract b² from both sides ]

step 4: (a + b)(a - b) = b(a - b) [ factor both sides ]

step 5: (a + b) = 1b [ after you divide both sides by (a - b) ]

step 6: 2b = 1b [ since a = b, (a + b) = 2b ]

step 7: 2 = 1 [ after you divide both sides by b ]

For a pint, anyone know which step is wrong, and why?

(Now corrected so that the squares look correct!)

Comments

Pencils down!

[jonnyargles.livejournal.com]

Well, not that I have a maths degree, but stage 2 proves that b =2, and step 1 states that a=b.

By that rationale the formulae should run.

1. a = 2

Therefore subsequently
3. 2x2 = 2x2 (tautology)

4. (2 + 2)(2 - 2) = 2(2-2) = 4 x 0 = 2 x 0

5. (2 + 2) = (1 x 2)

Four does not equal two. Um, so like Christie said, a-b = 0 and you can't divide by zero. Do I get more points for showing my working?

Re: Pencils down!

[sesquipedality.livejournal.com]

The 2s indicate "squared". It doesn't prove any such thing.

Re: Pencils down!

[robinbloke.livejournal.com]

My bad, should have used the appropriate tags to indicate squares, a pint for effort ;)