Divisible Product

If m m is an integer, then ( m + 1 ) ( m + 3 ) ( m + 5 ) (m+1)(m+3)(m+5) must be divisible by which of the following integers?

3 5 7 9

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3 solutions

  • If m 0 ( m o d 3 ) m\equiv 0(mod\quad 3) , then 3 ( m + 3 ) 3|(m+3) , so the given expression is divisible by 3 3 .

  • If m 1 ( m o d 3 ) m\equiv 1(mod\quad 3) , then 3 ( m + 5 ) 3|(m+5) , and the result follows.

  • If m 2 ( m o d 3 ) m\equiv 2(mod\quad 3) , then 3 ( m + 1 ) 3|(m+1) , anew.

Thus, the given product must be divisible by 3.

A single counterexample is enough to show that the expression is not necessary divisible by the other options.

Choose m 1 ( m o d 315 ) m\equiv 1(mod\quad 315) , then the given product is not divisible by neither 5, 7 nor 9.

Arjuna Sarathy
Jun 7, 2016

Product of any consecutive term in the sequence 2n or 2n+1 ( (2n+1 were,N>=2) and (2n were N>=3) N=no of terms) is divisible by 3.

Towhidd Towhidd
Jun 7, 2016

=(2016!-2015!)/2017=2015! (2016-1)/2017=2015! 2015

here 2015! Is divisible with 2017. then the reminder must be 2015.

I suppose your solution is irrelevant to the problem. :/

Tapas Mazumdar - 4 years, 8 months ago

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