Math Is Fun Forum
  Discussion about math, puzzles, games and fun.   Useful symbols: √ ∞ ≠ ≤ ≥ ≈ ⇒ ∈ Δ θ ∴ ∑ ∫ π -

Login

Username

Password

Not registered yet?

#1 2006-02-07 09:48:11

NoSash
Novice

Offline

Four Equals Five

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 (2006-02-07 09:57:55)

#2 2006-02-07 09:58:00

darthradius
Full Member

Offline

Re: Four Equals Five

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.


The greatest challenge to any thinker is stating the problem in a way that will allow a solution.
                                                             -Bertrand Russell

#3 2006-02-07 17:57:01

krassi_holmz
Real Member

Offline

Re: Four Equals Five

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 (2006-02-07 17:57:47)


IPBLE:  Increasing Performance By Lowering Expectations.

#4 2006-02-07 19:47:56

kempos
Full Member

Offline

Re: Four Equals Five

step 5. there you should have absolute value.

#5 2006-02-07 20:00:30

krassi_holmz
Real Member

Offline

Re: Four Equals Five

Yep.


IPBLE:  Increasing Performance By Lowering Expectations.

#6 2006-02-08 03:37:19

mathsyperson
Moderator

Offline

Re: Four Equals Five

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.


Why did the vector cross the road?
It wanted to be normal.

#7 2006-02-08 03:51:39

NoSash
Novice

Offline

Re: Four Equals Five

yeah it all makes sense, thanks guys

#8 2006-02-14 10:52:50

NoSash
Novice

Offline

Re: Four Equals Five

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.

Board footer

Powered by FluxBB