x = 2 4 8 − 1 Let the sum of the factors of x strictly between 5 and 10 be m. Let the remainder when 9 9 9 9 9 9 9 , 9 8 4 i s d i v i d e d b y m + 1 b e n Then find m+n
Given a number
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I've reported the problem, and I'll copy the reason here:
= = = 2 4 8 − 1 ( 2 2 4 − 1 ) ( 2 2 4 + 1 ) ( 2 8 − 1 ) ( 2 1 6 + 2 8 + 1 ) ( 2 2 4 + 1 ) ( 2 2 − 1 ) ( 2 2 + 1 ) ( 2 4 + 1 ) ( 2 1 6 + 2 8 + 1 ) ( 2 2 4 + 1 )
Therefore 5 is also a factor of x, and the answer would be 25. (m=22, n=3)
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Thanks, I have updated the question to say "strictly between".
Those who previously answered 25 have been marked correct.
An easy but good question!!
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For completeness, you should explain why there are no other factors if x between 5 and 10. So far, you have only shown that 7 and 9 are factors.
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We will be using a simple yet useful identity throughout the problem which is
a 2 - b 2 = (a+b)(a-b)
2 4 8 - 1
= ( 2 2 4 - 1)( 2 4 8 +1)
=( 2 1 2 - 1)( 2 1 2 +1)( 2 1 2 - 1)( 2 1 2 +1)
=( 2 6 - 1)( 2 6 +1)( 2 6 - 1)( 2 6 +1)( 2 6 - 1)( 2 6 +1)( 2 6 - 1)( 2 6 +1)
= d
Therefore, the factors between 5 and 10 are 7 and 9.
⇒ m =16
( 9 9 9 9 9 ) 9 9 9 8 4 ≡ ? (mod 17)
99999 ≡ 5 (mod 17)
but according to fermat's little remainder theorem
a p − 1 ≡ 1 (mod p) where p is prime and a is any positive integer...
( 9 9 9 9 9 ) 1 6 ≡ 1 (mod 17)
( ( 9 9 9 9 9 ) 1 6 ) 6 2 4 9 ≡ 1 (mod 17)
⇒ n =1
Therefore m+n = 16+1
= 1 7