Products, sums and divisors

6 6 is divisible by 6 6 .
6 × 6 6\times6 is divisible by 6 + 6 6+6 .
6 × 6 × 6 6\times6\times6 is divisible by 6 + 6 + 6 6+6+6 .

Is it true that 6 × 6 × × 6 n times \underbrace{6\times6\times\cdots\times6}_{n\text{ times}} is divisible by 6 + 6 + + 6 n times \underbrace{6+6+\cdots+6}_{n\text{ times}} for all positive integers n n ?

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12 solutions

Marta Reece
Jul 29, 2017

To prove that the pattern does not work for any n n , it is enough to give one counterexample.

6 × 6 × 6 × 6 × 6 6\times6\times6\times6\times6 is not divisible by 6 + 6 + 6 + 6 + 6 = 5 × 6 6+6+6+6+6=5\times6 because it is not a multiple of 5 5 .

...

The reason it seemed to work was because 6 + 6 = 2 × 6 6+6=2\times6 and 2 2 is a divisor of 6 6 . As is 3 3 in 6 + 6 + 6 6+6+6 .

Next 6 + 6 + 6 + 6 = 4 × 6 6+6+6+6=4\times6 , and 4 4 is a divisor of 6 × 6 6\times6 . It is 5 5 where it finally does not work.

Moderator note:

With this sort of problem, one thing that helps prevent being fooled is to rewrite the expressions in compressed form:

6 6 \frac{6}{6} 6 2 2 × 6 \frac{6^2}{2 \times 6} 6 3 3 × 6 \frac{6^3}{3 \times 6} 6 4 4 × 6 \frac{6^4}{4 \times 6} 6 5 5 × 6 \frac{6^5}{5 \times 6}

Just eyeballing the fifth expression is enough to note that there will be no "5" factor in the numerator.

Great job on showing why this not only fails, but also showing why it seemed to work.

@Venkatachalam J has generalized this result

Agnishom Chattopadhyay - 3 years, 10 months ago
Vu Vincent
Aug 7, 2017

The statement is:

Is it true that 6 n 6^n is divisible by 6 n 6n for a positive integer n n ?

Let A = 6 n 6 n A = \frac{6^n}{6n} . We're going to prove that A A is not always an integer. A = 6 n 6 n A = \frac{6^n}{6n} A = 6 n 1 n \Rightarrow A = \frac{ 6^{n-1} }{n}
A = 6 n 1 n = 2 n 1 3 n 1 n \Rightarrow A = \frac{ 6^{n-1} }{n} = \frac{ 2^{n-1}\cdot 3^{n-1} }{n}

Now we see that; for A A to always be an integer, n = 2 a , 3 b n = 2^a, 3^b or 2 a × 3 b 2^a \times 3^b for some a , b n 1 a,b \le n-1 because if so, it will divide with the 2 n 1 2^{n-1} or 3 n 1 3^{n-1} or 6 n 1 6^{n-1} on the numerator. Since n n is of all positive integers, it can not just be of the form n = 2 a , 3 b n = 2^a, 3^b or 2 a × 3 b 2^a \times 3^b . An example would be n = 5 n = 5

Therefore, 6 n 6^n is not divisible by 6 n 6n for n Z + \forall n \in \mathbb{Z}^+

Nice, proof-based answer.

Zach Abueg - 3 years, 10 months ago

So, for exactly which values of n n does this work?

Agnishom Chattopadhyay - 3 years, 10 months ago

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2 a 2^a , 3 b 3^b or 2 a × 3 b 2^a \times 3^b for some a , b n 1 a,b \le n-1

Vu Vincent - 3 years, 10 months ago

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n=12 and n=18 work as well. Combinations of powers of 2 and powers of 3 would appear to be excluded from the proposed solution set.

Kevin Weatherwalks - 3 years, 10 months ago

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@Kevin Weatherwalks thanks for the comment ill fix it :)

Vu Vincent - 3 years, 10 months ago
Venkatachalam J
Aug 7, 2017

In general, the pattern will not work for the prime number of times excluding 2 and 3.

6^(prime number of times other then 2 and 3) will not divisible by (prime number of times other then 2 and 3)*6

with three it is possible but it's impossible for other primes other than 2 and 3

Gian Carlo - 3 years, 10 months ago

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Thank you. I updated the solution.

Venkatachalam J - 3 years, 10 months ago

I like this explanation. It might generalize further to any number n that has a non-2 or non-3 factor. So 6^n will not be divisible by n*6 for n={5, 7, 10, 11, 13, 14, 15, ...}.

Andrew Lamoureux - 3 years, 10 months ago

So I guess you could just say that it will work if n does not have any prime factors other than 2 and 3 (so this includes all primes other than 2 and 3, any coprimes of 6 (e.g. 25) and the rest.

Hanuman Bajrangbali - 3 years, 10 months ago
Mohammad Khaza
Aug 8, 2017

we don't have to go far,

6 1 , 6 2 , 6 3 , 6 4 6^1 ,6^2 ,6^3 ,6^4 all are divisible according to the condition.

but, 6 5 6 + 6 + 6 + 6 + 6 \frac{6^5}{6+6+6+6+6} = 1296 5 \frac{1296}{5}

as, 6 5 6^5 is not multiple of 5,so it is not divisible by 30(multiple of 5)

Good work! We just need to find a example to disprove a "Is it true for all n?" statement.

Pi Han Goh - 3 years, 10 months ago
Jesse Nieminen
Aug 7, 2017

Counterexample: n = 5 n = 5 . Hence, No \boxed{\text{No}} .

Such a short solution haha! I guess it's not much of a challenge if the counterexample is too small.

What would the counterexample be if we replace all the 6's with the gigantic number lcm ( 1 , 2 , 3 , , 1 0 1 0 100 ) \text{lcm} \left (1,2,3,\ldots , 10^{10^{100}} \right) ?

Pi Han Goh - 3 years, 10 months ago

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n = 2 74207281 1 n = 2^{74207281} - 1 ;)

Fine, I'll do a proper solution.

Jesse Nieminen - 3 years, 10 months ago

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Actually, I have problem in mind and my solution would ruin it.

Jesse Nieminen - 3 years, 10 months ago

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Alex Sedlmayr
Aug 12, 2017

The prime factors of 6 n 6^n are only 2 and 3 for any n; For any number n which has a different prime factor (e.g. n = 5, 7, 10, 11, 14,...) the number n × 6 n \times 6 has this prime factor as well, so 6 n 6^n is not divisible by it in those cases.

Yup, that's the reason why I chose the number 6. Good work!

Pi Han Goh - 3 years, 10 months ago
Devashish Tomar
Aug 11, 2017

6^n-1/(n) is not for every positive integer

Yes, but can you prove it?

Pi Han Goh - 3 years, 10 months ago
Robert DeLisle
Aug 10, 2017

Not even close. The divisor is 6 x n Powers of 6 have only factors 2 and 3. The power of 6 will stop being divisible by 6 x n as soon as n has a factor that is not 2 or 3, at n=5.

Awhhh, it's not challenging if the counterexample is too small right?

If I were to set another question, what number should I replace (all the) 6's with so that the counterexample is large?

Pi Han Goh - 3 years, 10 months ago
Akshay Gupta
Aug 9, 2017

Up to 4 terms it seems favorable but as we calculate with 5 terms, we the answer NO

6 x 6 x 6 x 6 x 6 and 6 + 6 + 6 + 6 + 6

6 x 6 x 6 x 6 x 6 and 6 ( 1 + 1 + 1 + 1 + 1 )

6 x 6 x 6 x 6 x 6 and 6 x 5

Hence, answer is NO

Yup, you can just say that 6^5 is not divisible by 5, so 6^5 is not divisible by (6x5=30) either. +1

Pi Han Goh - 3 years, 10 months ago
Ray Birklid
Aug 8, 2017

This is actually a poorly worded question. As the question is phrased the answer is yes. If the words 'is divisible' in the question were reworded to 'is evenly divisible' then the answer would be no.

I don't think that's necessary.

Are you saying that all the statements "x is divisible by y" should be replaced with "x is evenly divisible by y" ?

Pi Han Goh - 3 years, 10 months ago
Eeshan Khan
Aug 7, 2017

Any number is not divisible by a prime number not in its prime factorisation. Here, for a number not,we are basically checking 6^n/(6n). We get 6^(n-1)/n. =2^(n-1)*3^(n-1)/n. If n is a prime number other than 2 or 3, it is indivisible.

Wait, this mean that 6^4 is not divisible by (6 x 4) either. Do you know where you went wrong?

Pi Han Goh - 3 years, 10 months ago
Rudra Jadon
Aug 7, 2017

Here we have the form 6×6×6.....(n times)/(6+6+6......{n times}) so, we can simplify it to 6^n/(6n) so here we can see it is written as 6^(n-1)/n so if 6^(n-1) is divisible by n then it is true but 6^(n-1) is not always divisible by n hence the answer is NO

Yay! For completeness, you should provide a counterexample, like (n=7).

Pi Han Goh - 3 years, 10 months ago

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