Four Equals Five

NoSash · 2006-02-06 10:48:11

1) 16 - 36 = 25 - 45
2) 16 - 36 + 81/4 = 25 - 45 + 81/4
3) 4^2 - 2x4x9/2 + (9/2)^2 = 5^2 - 2x5x9/2 + (9/2)^2
4) (4 - 9/2)^2 = (5 - 9/2)^2
5) 4 - 9/2 = 5 - 9/2
6) 4 = 5

What went wrong?

I'm pretty sure the error is from step 3 to 4 but I don't know what the heck they are trying to do.

Last edited by NoSash (2006-02-06 10:57:55)

darthradius · 2006-02-06 10:58:00

when you take the square root, you need the ± which gives you a few different combinations...this "proof" just uses one combination that does not work.

krassi_holmz · 2006-02-06 18:57:01

e.a.
4) (4 - 9/2)^2 = (5 - 9/2)^2
5) |4-9/2|=|5-9/2|
6) |-0.5|=|+0.5|
which is OK.

Last edited by krassi_holmz (2006-02-06 18:57:47)

kempos · 2006-02-06 20:47:56

step 5. there you should have absolute value.

krassi_holmz · 2006-02-06 21:00:30

Yep.

mathsyperson · 2006-02-07 04:37:19

All of the above is correct. They've hidden it with lots of unnecessary complications, but what they're basically doing is saying that as x² = (-x)², then x = -x, even though it doesn't.

NoSash · 2006-02-07 04:51:39

yeah it all makes sense, thanks guys

NoSash · 2006-02-13 11:52:50

Hey guys, apparently it's incorrect to put the absolute value signs, can anybody confirm this? Or support it? I know it works out mathematically but I was told it's still wrong.

Math Is Fun Forum

#1 2006-02-06 10:48:11

Four Equals Five

#2 2006-02-06 10:58:00

Re: Four Equals Five

#3 2006-02-06 18:57:01

Re: Four Equals Five

#4 2006-02-06 20:47:56

Re: Four Equals Five

#5 2006-02-06 21:00:30

Re: Four Equals Five

#6 2006-02-07 04:37:19

Re: Four Equals Five

#7 2006-02-07 04:51:39

Re: Four Equals Five

#8 2006-02-13 11:52:50

Re: Four Equals Five

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