Four equals to three

Algebra Level 2

Kimmie wants to prove that 4 = 3 4=3 .

She starts off with the equation a b = c a-b=c which is equivalent to

4 a 3 a 4 b + 3 b = 4 c 3 c \large 4a-3a-4b+3b = 4c-3c

She moved the variables to obtain 4 a 4 b 4 c = 3 a 3 b 3 c 4a-4b-4c = 3a-3b-3c and factorized it to

4 ( a b c ) = 3 ( a b c ) \large 4(a-b-c) = 3(a-b-c)

Finally, she cancelled both sides to obtain 4 = 3 4=3 .

What is the error in Kimmie's work?

Image credit: IIT Bombay.
She made an error factoring the terms. She divided by 0. She made an arithmetic error whilst moving variables. Kimmie is correct.

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48 solutions

Aman Jaiswal
Feb 1, 2014

If a-b=c then it is obvious that a-b-c=0. For example x-3y=6 implies that x-3y-6=o

I like how you used 8 words to come with a solution that explained the whole problem. Nice work!

Kevin Mo - 7 years, 4 months ago

a-b-c=0 We can not divide by zero

ARUNA MANE - 7 years ago

Nice job!

Valerian Pratama - 7 years, 4 months ago

That's why they call it Maths! :D So, trivial yet so brain-wrecking!

Thanks for the problem Kevin. ;)

Kou$htav Chakrabarty - 7 years, 3 months ago

Good Paradox

Adhiraj Mandal - 7 years, 3 months ago

Great Explanation!!

Dhiren Shah - 7 years, 2 months ago

good!!!

Rajarshi Majumder - 7 years, 2 months ago

a-b=c ....then a-b-c=0 .... and something divided by 0 is not possible

Asif Xaman - 7 years, 1 month ago

Yah!

Mardokay Mosazghi - 7 years, 4 months ago

Softly Busted :D

Karim Monir - 7 years, 4 months ago

simple yet interesting to solve...!!!!

sunitha p - 7 years, 3 months ago

aawsum..!!!!

Adhi Anand - 7 years, 2 months ago

according to Bodmas rule it must be 4th option as this would be the eqn 4a-3a-7b=4c-3c can any one xplain me in this?

Sandeep Vishwamithra - 6 years, 10 months ago

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Mdas may be circumvented as multiplication and division orders don't have a consequence, neither do addition and subtraction. But to make this easier to see, think of the equation as (4a)+(-3a)+(-4b)+(3b)=(4c)+(-3b)

Seth Boden - 5 years, 10 months ago

WHA......! Great solution!

Ruhan Habib - 7 years, 3 months ago
Prasun Biswas
Feb 8, 2014

The starting equation states that ----

a b = c a b c = 0 a-b=c \implies a-b-c=0 .

Now, when Kimmie cancelled both the sides in her work, what she actually did was dividing both sides of the equation 4 ( a b c ) = 3 ( a b c ) 4(a-b-c)=3(a-b-c) by ( a b c ) (a-b-c) so as to get 4 = 3 4=3 but since ( a b c ) = 0 (a-b-c)=0 , so she actually divided both sides of the equation by 0 0 which is mathematically incorrect.

So, her main error in the work was Dividing both sides by 0.

You know, lots of people without formal algebra education (i.e. knowledge that you can't divide by zero) make this mistake when trying to prove impossible things. Another mistake that is often made is taking the square root of negative number without using the formal definition of i (square root of minus 1).

Justin Chan - 7 years, 3 months ago

Since she has assumed a-b=c,but in the end 4(a-b-c)=3(a-b-c) makes 4(0)=3(0) therefore both sides multiplied by 0 (her assumption is wrong to prove 4=3. K.K.GARG,INDIA

Krishna Garg - 7 years, 3 months ago

Not main error but the only error

Kushagra Sahni - 7 years, 3 months ago

good thinking

Abhay Rajput - 7 years, 4 months ago

Cool (Y)

Nilesh Mali - 7 years, 4 months ago

good solution

Ashwin Upadhyay - 7 years, 4 months ago

ooooooo.....

Tootie Frootie - 7 years, 3 months ago

good solution

KHANDAKER TAWHEED - 7 years, 3 months ago

good dear ..

Sambit Sahu - 6 years, 9 months ago

Good soln.

Ashley Shamidha - 7 years, 1 month ago
Kevin Mo
Feb 1, 2014

The answer is obvious. Division by 0 is strictly prohibited, which causes no solution already. No solution means that 4 != 3.

AH! I was stuck at this the whole day. But great question, Kevin. Really stimulates the mind into working.

Arjun Bahuguna - 7 years, 3 months ago

Since from the original equation a b = c a - b = c , then a b c = 0 a - b - c = 0 . In the factoring, she divided by a b c = 0 a - b - c = 0 , and thus, that is the answer.

Aryaman Singh
Feb 7, 2014

lets start with -

a - b = c

a - b - c = 0

4 ( a - b - c ) = 4 ( 0 ) = 0 --- eqn (1)

3 ( a - b - c ) = 3 ( 0 ) = 0 ---eqn (2)

dividing eqn. 2 by eqn 1 ,

4 ( a- b - c ) / 3 ( a - b - c ) = 0 / 0

Here , Kimmie took 0 / 0 to be equal to 1, thereby getting 4 = 3 as the result.

However , as we know, 0 / 0 is not defined , this was a wrong step. Hence the answer.

a-b=c so, a-b-c=0

4(a-b-c)=3(a-b-c) because a-b-c = 0, then 4 (a-b-c) = 3 (a-b-c) should not be divided by (a-b-c) or in other words should not be divided by 0.

niceanswer

Aswad Hariri Mangalaeng - 7 years, 4 months ago
Raghav Bakshi
Feb 8, 2014

Okay jump to the last equation

4(a - b - c) = 3(a - b - c)

4(a - b - c) - 3(a - b - c) = 0

(a - b - c)( 4- 3 ) = 0

Therefore a - b - c has to be = 0

* And therefore in the last step we cannot cancel by a - b - c as it would be equivalent to dividing by 0 *

Victor Loh
Feb 8, 2014

a b = c a-b=c a b c = 0 a-b-c=0 Since you cannot divide by 0, the conclusion is proven.

Dilbwag Singh
Feb 7, 2014

By the problem if 4(a-b-c)=3(a-b-c). But earlier she started with a-b=c; or this can also be written as a-b-c=0. Therefore 4(a-b-c)=3(a-b-c), which can also be written as 4(0)=3(0). Hence she divided by 0 is the correct answer.

Pariah Dimasupil
Mar 3, 2014

Aman Jaiswal is right. Obviously, a b c = 0 a-b-c=0 .

Khaled Mohamed
Feb 11, 2014

a major mistake, a-b=c then a-b-c=0 , and any constant divided by 0 equals undefined (or infinity)

Kimmie took a-b=c, so a-b-c=0. At the final stage She wrote 4(a-b-c)=3(a-b-c), that is 4 0=3 0 which gives 0=0, she can't divide both sides by (a-b-c) for cancellation, as it is 0. It was the main error that she divided by 0.

Hùng Minh
Feb 10, 2014

We have a - b = c <=> a - b - c = 0. Kimmie cannot both sides 4(a - b - c) = 3(a - b - c).

Ashutosh Jha
Feb 10, 2014

when the equation turned to, 4(a-b-c)=3(a-b-c) we look back to the guiding equation, a-b=c, which yields a-b-c=0 which means we are dividing by zero.

Subhrajyoti Sinha
Feb 10, 2014

as a-b=c the a-b-c=0 so it becomes 0/0 which is incorrect

its called divide by zero exception in algebra, step 1 says [a-b=c that is a= b+c ] and second last step 4[a-b-c]=3[a-b-c] replacing a-b with c , we get 4[c-c]=3[c-c] that is 4x0=3x0 , last step we "divide by zero" as 4x0/0= 3x0/0 which is not legit.

Raghav Dua
Feb 9, 2014

If we solve this the usual algebraic way, we won't find anything unusual. But this Mathematical fallacy obviously calls for a little more careful analysis.

If you see the first step itself ( a - b = c ) and shift c to the left side, it becomes a - b - c = 0

So practically in the final step, you are dividing by 0! And that also without realizing because we always try to eliminate as many unknowns as possible.

Benjamin Wong
Feb 9, 2014

a-b-c=0 because a-b=c, so moving c to the left makes a-b-c=0.

Dilshad Digonta
Feb 8, 2014

she assumed a-b=c →a-b-c=0 in the final step she cancelled (a-b-c) from both sides. but (a-b-c)=0

Saurabh Pranjale
Feb 8, 2014

a - b = c

a - b - c = 0

Vinayak Raj
Feb 8, 2014

As we can see that a-b=c , therefore a-b-c=0 and we cannot cancel the terms on both sides, which results to zero.

She assumed that (a-b=c) which implies (a-b-c=0) , later she divided( a-b-c ) on both the sides and there lies the error.

Abdullah Al Mamun
Apr 25, 2016

she did the mistake at first line. because.

a - b = c

and,

a - b - c =0

when she try to clear both side with (a - b - c) , actually she divided with 0. and 0/0 is no valid.

Luke Limbo
Sep 4, 2015

Aaa... Ok, I got it.... When she factored it out, the statement 4(a-b-c)=3(a-b-c) became 4(0)=3(0) then she divided it both sides by zero...

Prajwal Kavad
Apr 7, 2014

a-b=c hence a-b-c=0 therefore mistake is that it was divided by zero

Vaishnavi Gupta
Mar 24, 2014

Now this one was pretty simple, folks. The first assumption itself says : a-b=c, which means that a-b-c=0 . Whereas in the last step, the same term: a-b-c is cancelled out, but we cant cancel out zero. So, that is the error!

Piyush Sarwariya
Mar 9, 2014

a-b=c
therefore
a-b-c=0

Shashank Sharma
Mar 7, 2014

a-b-c = 0

Nikky Fauzdar
Mar 2, 2014

a-b-c=0 so divide by 0 got it error

Brahmam Meka
Feb 28, 2014

you cannot cancel a-b-c as that is zero

Bikash Kumar
Feb 27, 2014

4th step is incorrect because from 1st step; a-b=c;which implies a-b-c=0 and the error in the first step is we are dividing a-b-c/a-b-c=0/0 which doesnot exist

if a-b = c a-b-c = 0 so, it cannot be cancelled out.

Rakshit Pandey
Feb 23, 2014

Given: (a-b)=c. Replace (a-b) with c before cancelling out in last step. We get 4(0)=3(0). :P

Khalid Masum
Feb 23, 2014

a - b = c so a - b - c = c - c = 0

Nothing can be divided by 0. So it is the main error of Kimmie,s.

Arijeet Satapathy
Feb 23, 2014

The starting equation states that ----

.

Now, when Kimmie cancelled both the sides in her work, what she actually did was dividing both sides of the equation by so as to get but since , so she actually divided both sides of the equation by which is mathematically incorrect.

So, her main error in the work was Dividing both sides by 0.

Ravi Saini
Feb 21, 2014

We start with a-b = c;

This means a-b-c = 0.

In last step both sides are divided by a-b-c, which is zero division and in valid

Manpreet Kaur
Feb 21, 2014

since a-b=c implies a-b-c=0 how can she divide an equation with zero?? thats the error....

Tauhid Shammo
Feb 19, 2014

a-b=c

a-b-c=0

4(a-b-c)=3(a-b-c)

4(0)=3(0)

0=0

She devided by 0

Abhishek Jha
Feb 19, 2014

it's easy, as a-b=c; thus a-b-c=0 and if both the sides of an equation carry 0, we cannot change their sides as by changing sides the finite equation will become infinite and incorrect.

Shahriar Ahmed
Feb 18, 2014

a-b=c so a-b-c=0

Srijan Neogi
Feb 17, 2014

IN THE 4th STEP SHE WROTE : 4(a-b-c) = 3(a-b-c) NOW , SHE DIVIDED BOTH THE SIDES BY (a-b-c). BUT SHE ASSUMED THAT a-b =c => a-b-c = 0 THEREFORE SHE DIVIDED BOTH THE SIDES BY 0.

Oliver Daniel
Feb 17, 2014

After the factorization 4(a-b-c) = 4((a-b)-c) = 4(c-c) since a-b=c. as such the L.H.S. = 4x0 and R.H.S is 3x0. 4=3 can only be obtained if there is a division by zero.

Asif Mohammad Omi
Feb 15, 2014

She started off with the equation a-b=c =>a-b-c=0

but at the end she got 4(a-b-c)=3(a-b-c) =>4x0=3x0 =>0=0

But before that she cancelled a-b-c from both side which is impossible. Because for this she need to divide by 0 in both sides, which is impossible. So this is wrong.

Shikhar Jaiswal
Feb 15, 2014

she shouldnt have cancelled in the last step!!

Josh Grotstein
Feb 15, 2014

Since a-b=c, Then (a-b-c) = 0

Saradindu Hui
Feb 13, 2014

Here a-b=c,so,a-b-c=0,so.we can not cancel both side (a-b-c) in last step.

To cancel any variable term in an algebraic eqation, it is necessary to make sure that the term which is being cancelled is not zero...as dividing on both the sides by zero is INDETERMINATE form..

Mohamed Sajjadh
Feb 12, 2014

a-b=c a-b-c=0 so at last step we cannot cancel (a-b-c) terms because its indeterminate number '0' (i.e.) its logical error

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