Ok Math whiz kids...where is the error?

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Ok Math whiz kids...where is the error?

Postby Bilnick » Fri Oct 10, 2008 1:18 pm

9 = 8

This proof, provided by Michigan State assistant math professor Ignacio Uriarte-Tuero, would seem to prove that, in fact, nine does equal eight, as Tampa Bay Rays manager Joe Maddon has asserted all year.

It doesn’t, of course, as nine can’t equal eight. Though the error is difficult to spot. Can you find it?

Step 1: Let x = y. Multiply by y to get
xy = y2

Step 2: Subtract x2 to get
xy − x2 = y2− x2

Step 3: Factoring, we get
x(y − x) = (y − x)(y + x)

Step 4: which simplifies to
x = y + x

Step 5: and now using that x = y we get
x = 2x

Step 6: Simplifying again, we get
1 = 2

Step 7: so that when we add 7 to both sides we get
8 = 9

Step 8: or put in other words,
9 = 8


There is an error in this proof, obviously...but I couldn't find it....can you??
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Re: Ok Math whiz kids...where is the error?

Postby ThinkInk » Fri Oct 10, 2008 1:28 pm

It's been a very long time since I've worked on any problems as this and currently I couldn't add 1+2 and come up with the correct answer without using a calculator but, x is not automatically 1 and 2y isn't 2.

But it looks good on paper!
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Re: Ok Math whiz kids...where is the error?

Postby Fenina » Fri Oct 10, 2008 1:41 pm

Ack was trying to run that through my head and it kept coming up with err divide by zero, this thought has crashed.
I have no idea really, feels like something around step 2 to step 3.
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Re: Ok Math whiz kids...where is the error?

Postby Jahras » Fri Oct 10, 2008 1:54 pm

if x=y, then he divides by zero when he simplifies. which is a big no-no!
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Re: Ok Math whiz kids...where is the error?

Postby Bilnick » Fri Oct 10, 2008 1:56 pm

Thank you!
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Re: Ok Math whiz kids...where is the error?

Postby Goofydoofy » Fri Oct 10, 2008 2:25 pm

Keep your math to youself!
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Re: Ok Math whiz kids...where is the error?

Postby Chadeaux » Fri Oct 10, 2008 4:19 pm

Goofydoofy wrote:Keep your math to youself!


unless your counting goats?
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Re: Ok Math whiz kids...where is the error?

Postby Goofydoofy » Fri Oct 10, 2008 5:21 pm

I got better things to do to goats then count em!
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Re: Ok Math whiz kids...where is the error?

Postby Horcrux » Sat Oct 11, 2008 3:16 pm

Bah, wish I saw this when it was posted :(

I should think of a math problem to post to yal!
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Re: Ok Math whiz kids...where is the error?

Postby Bilnick » Mon Oct 13, 2008 11:33 am

One of my favorite math questions to ask people is simple addition, but a big majority of people will answer wrong. But you must ask them orally, no paper and pencil or calculator allowed.

Tell someone to start with 1000

then add 40

then add 1000

then add 30

then add 1000

then add 20

then add 1000

then add 10















what did you get.............5000?



















That is wrong. But it is the most common answer.
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Re: Ok Math whiz kids...where is the error?

Postby Goofydoofy » Mon Oct 13, 2008 12:08 pm

Yea, that is a good one. Should be 4100. Ain't the brain a wonderful thing?
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Re: Ok Math whiz kids...where is the error?

Postby Hilf » Mon Oct 13, 2008 6:31 pm

damn that hurt my pride cause i got that wrong reading it
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Re: Ok Math whiz kids...where is the error?

Postby Chadeaux » Mon Oct 13, 2008 9:24 pm

what is scary is Goofy knows it :P
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Re: Ok Math whiz kids...where is the error?

Postby EQIsenhart » Thu Oct 16, 2008 1:01 pm

Hmmm, Bilnick I got it right, 4100, I am trying to see how people would get 5000? I don't see it
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Re: Ok Math whiz kids...where is the error?

Postby Goofydoofy » Thu Oct 16, 2008 1:15 pm

Throwing the 1000 in there every other number gets the brain thinking in 1000s. So, when you add the 100 up, the mind thinks 1000 instead. Now, we know you don't have a normal brain, so you naturally got the answer correct.
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