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Re: Sum of all positive factors of N VS 8232 [#permalink]
himanshu13 wrote:
answer is C, but I am not sure what's the logic.
I could do only with calculator


Did you use a Scientific Calculator??
;)
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Re: Sum of all positive factors of N VS 8232 [#permalink]
KarunMendiratta wrote:
himanshu13 wrote:
answer is C, but I am not sure what's the logic.
I could do only with calculator


Did you use a Scientific Calculator??
;)



Nope, I did the subtraction and then found the factors by hit and trial, difference was divisible by 5 then resultant by 13 then 5 again
Didn't take much time though

Initially I was trying to find the pattern with initial powers of 3 and 2, but no success.
What is the solution?
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Sum of all positive factors of N VS 8232 [#permalink]
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grenico wrote:
We can use the difference of squares to solve this easily.

Note that for any integer \(n\), we have:

\((a^{n} - b^{n}) = (a^{n/2} - b^{n/2})(a^{n/2} + b^{n/2})\)

So we have:

\((3^{8} - 2^{8})\)

\((3^{4} - 2^{4})(3^{4} + 2^{4})\)

\((3^{2} - 2^{2})(3^{2} + 2^{2})(3^{4} + 2^{4})\)

\((3 - 2)(3 + 2)(3^{2} + 2^{2})(3^{4} + 2^{4})\)

\((1)(5)(9+4)(81+16)\)

\((1)(5)(13)(97)\)

Since 97 is prime, we can no longer factor this expression.

Now we need to multiply the different combinations of these factors to get the complete list of factors for \((3^{8} - 2^{8})\).

Think of factoring 12. This would be \(2^{2} * 3\), or \(4*3\). Summing 4 and 3 isn't enough, we're forgetting the 1,2,6 and 12.

Permuting the different factors then, we get:

\(1,5,13,97,5*13,5*97, 13*97, 5*13*97\)

Which simplifies too (I used a calculator here, definitely saves time):

\(1,5,13,97,65, 485, 1261, 6305\)

Summing these we get \(8232\).

Therefore the answer is C


In order to find the sum of factors of any number \(N\), where
\(N = a^xb^yc^z.....\) and \(a, b, c\) are prime factors

Sum of (+ve) factors = \((\frac{a^{x+1} - 1}{a - 1})(\frac{b^{y+1} - 1}{b - 1})(\frac{c^{z+1} - 1}{c - 1})..\)

Here, \(N = (5)(13)(97)\)

Sum = \((\frac{5^2 - 1}{5 - 1})(\frac{13^2 - 1}{13 - 1})(\frac{97^2 - 1}{97 - 1})\)

= \(\frac{(24)(168)(9408)}{(4)(12)(96)} = 8232\)
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Re: Sum of all positive factors of N VS 8232 [#permalink]
is there an easier way to solve this? The explanations given in this thread isn't too helpful
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Re: Sum of all positive factors of N VS 8232 [#permalink]
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snodoodles234 wrote:
is there an easier way to solve this? The explanations given in this thread isn't too helpful


I'd be very surprised to find this exact question on the GRE, to be honest. I could see a question with the same concepts (e.g. recognizing the difference of squares as noted above, and finding the factors that way), but not a question in which you'd need to reach a high sum like 8232. There's much more "busy work" on this question than in most official GRE questions, IMO.
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Re: Sum of all positive factors of N VS 8232 [#permalink]
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I believe that Karun did questions at the time quite hard for the GRE. However, the same concepts are tested. As such, for the exam this is still a valuable training
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Re: Sum of all positive factors of N VS 8232 [#permalink]
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snodoodles234 wrote:
is there an easier way to solve this? The explanations given in this thread isn't too helpful



I do not see the following explanation

Quote:
In order to find the sum of factors of any number \(N\), where
\(N = a^xb^yc^z.....\) and \(a, b, c\) are prime factors

Sum of (+ve) factors = \((\frac{a^{x+1} - 1}{a - 1})(\frac{b^{y+1} - 1}{b - 1})(\frac{c^{z+1} - 1}{c - 1})..\)

Here, \(N = (5)(13)(97)\)

Sum = \((\frac{5^2 - 1}{5 - 1})(\frac{13^2 - 1}{13 - 1})(\frac{97^2 - 1}{97 - 1})\)

= \(\frac{(24)(168)(9408)}{(4)(12)(96)} = 8232\)


quite hard

See more here to reinforce your concepts https://gre.myprepclub.com/forum/gre-ma ... tml#p95905
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