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hmm Maths, Physics etc thread...

Alex

Alex

sKIp_E
Staff member
Administrator
Super Moderator
Ok, a mathsy problem....Ive got two, Ill give the really simply one first, simply because otherwise we'd be here all day...

PLEASE, if you're a smart person dont just come in straight away with the answer...Sit on it for a while...

We'll start with the simple statement:

(A): x^2-x^2 = x^2-x^2 (for those that dont know, ^ is to the power of)

Ok lets take the LHS:
Take out the common factor "x"
x(x-x)

RHS
Using the difference of two squares method of factorisation:
(x-x)(x+x)


From (A):
LHS=RHS
x(x-x)=(x-x)(x+x)

Now cancelling the common (x-x), we're left with:

x=(x+x)
x=2x

Cancel the common "x"
1=2

But!?


Prove me wrong, I know the mistake, do you? (NO SMART PEOPLE REPLY TO SOON! OR OLL GIVE YOU THE HARD ONE!)
 
M

Moron

Fast Breeder
Life Ban
I don't give a **** about Math and Physics


as long as i get a B in both classes


Im happy (Y)


;)
 
I

industrie

Youth Team
Originally posted by Alex


x=(x+x)
x=2x
Click to expand...

damn u ben... i was just about to say that...

substitute any number for x ... ie

x^2-x^2 = x^2-x^2 (where x = 3)

9-9 = 9-9 thus equals 0
-----------
(3-3)(3+3)

9+9-9-9 thus equals 0
------------

x(x-x)=(x-x)(x+x)
3(3-3) = (3-3)(3+3)
9-9= 9+9-9-9 hence 0 again

-----------
substituting 3 for x still
x=(x+x)
3=(3+3)
3=9 (big no!!!)
can only be zero
-------------
x=2x
3=2.3
3=6 NO no no... impossibile signore...
must be zero...

thus... x equals 0
 
Alex

Alex

sKIp_E
Staff member
Administrator
Super Moderator
You guys have proved nothing...

Youve both proved that at the beginning the thing equals zero...And youve subbed in real numbers to find where the error occurs, BUT you havent given me a mathematical reason for the error...

Follow the x's and there is seemingly no mistake. Find one, and tell me WHAT it is!
 
Alex

Alex

sKIp_E
Staff member
Administrator
Super Moderator
Talking to Industrie on MSN, his mistake was he kept comparing the two sides...Doing that is no use...You cant just say "x cant equal 2x unless x=0", because that is exactly what you have to prove...
If x cant equal 2x, then why does my factorising show that it can...
 
I

industrie

Youth Team
x=2x

hmmm
well of course there is still a common x here... so if i expand it again... so that x= (x+x)

so cancel another x from each side leaving the LHS with nothing and the RHS with a simple x... thus meaning x = 0....


but the reason...
 
Alex

Alex

sKIp_E
Staff member
Administrator
Super Moderator
Originally posted by industrie
x=2x

hmmm
well of course there is still a common x here... so if i expand it again... so that x= (x+x)

so cancel another x from each side leaving the LHS with nothing and the RHS with a simple x... thus meaning x = 0....


but the reason...
Click to expand...
NO!

Get off this whole x=0 thing, it doesnt prove anything...

Insert this above the original proof....

x = any REAL number...

x doesnt have to equal anything, the steps Ive taken to reach the conclusion are simply algebraic steps...So x doesnt neccesarily equal 0.
 
A

AhmedK

Min
i agree!!

Insert this above the original proof....

x = any REAL number...

x doesnt have to equal anything, the steps Ive taken to reach the conclusion are simply algebraic steps...So x doesnt neccesarily equal 0.

well of course there is still a common x here... so if i expand it again... so that x= (x+x)

so cancel another x from each side leaving the LHS with nothing and the RHS with a simple x... thus meaning x = 0....


but the reason.Doing that is no use...You cant just say "x cant equal 2x unless x=0", because that is exactly what you have to prove...
If x cant equal 2x, then why does my factorising show that it can...
 
I

industrie

Youth Team
Originally posted by AhmedK
i agree!!

Insert this above the original proof....

x = any REAL number...

x doesnt have to equal anything, the steps Ive taken to reach the conclusion are simply algebraic steps...So x doesnt neccesarily equal 0.

well of course there is still a common x here... so if i expand it again... so that x= (x+x)

so cancel another x from each side leaving the LHS with nothing and the RHS with a simple x... thus meaning x = 0....


but the reason.Doing that is no use...You cant just say "x cant equal 2x unless x=0", because that is exactly what you have to prove...
If x cant equal 2x, then why does my factorising show that it can...
Click to expand...

but it still doesnt prove it... u've just echoed what im saying in writing...
 
Alex

Alex

sKIp_E
Staff member
Administrator
Super Moderator
Nah ahmed is agreeing with me, in saying that you proved nothing...
 
I

industrie

Youth Team
im proving the """"incorrenctness"""" of ur formula is correct :p

hmmmmm.... tisk tisk tisk... its been a while since i've done school maths
 
Y

yim87

Senior Squad
i'm guessing this.

as x = x, therefore x-x = 0. so, u can't say x and (x + x) is the same as any number when multipied by x-x (which is 0) will get the same answer...

if u say x = x + x , it's the same as saying 67 = -514 as

67 ( x - x ) = - 514 ( x - x )
 
Alex

Alex

sKIp_E
Staff member
Administrator
Super Moderator
Yeah yim87 is basically correct in what he said...If you put together what he said, and what Rhizome said, you get the answer...

From teh step
x(x-x)=(x-x)(x+x)
to x=x+x

u're cancelling (x-x) which is 0. By cancelling, u're dividing, and you cant divide by 0...
 
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