Find The Remainder

Find the remainder when 999 ! × 1011 ! 999! \times 1011! is divided by 2011. 2011.

Details and assumptions

  • You might use the fact that 2011 2011 is a prime.


The answer is 1.

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4 solutions

Andrea Ventola
Jun 2, 2014

1011 1000 ( m o d 2011 ) 1010 1001 ( m o d 2011 ) 1009 1002 ( m o d 2011 ) 1 2010 ( m o d 2011 ) 1011 \equiv -1000 \pmod {2011} \\ 1010 \equiv -1001 \pmod {2011} \\ 1009 \equiv -1002 \pmod {2011} \\ \vdots\\ 1 \equiv -2010 \pmod {2011} \\ So 1011 ! × 999 ! 2010 ! ( m o d 2011 ) 1011! \times 999! \equiv -2010! \pmod {2011} \\ and for Wilson's Theorem 1011 ! × 999 ! 1 ( m o d 2011 ) 1011! \times 999! \equiv 1 \pmod {2011}

How did you get your 5th line?.Please explain to me in details. And what is Wilson's theorem?

Anik Mandal - 7 years ago

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Dude....how did you solve this problem??????

Krishna Ar - 7 years ago

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By Utkarsh Rajput's solution, mate!

Anik Mandal - 7 years ago
Utkarsh Rajput
Jun 3, 2014

I came up a different way but I also need your help for making it true, l e t c o n s i d e r x + y = p 1 w h e r e p i s p r i m e , w h e n x ! × y ! i s d i v i d e d b y p h e r e s I n e e d y o u r h e l p , i t i s q u i t e o b v i o u s t h a t b o t h x a n d y w i l l b e o d d s i m u l t a n e o u s l y o r e v e n , t h e r e f o r e w h a t i s h y p o t h e s i z e d t h a t x , y a r e o d d r e m a i n d e r i s 1 a n d w h e n e v e n r e m a i n d e r i s 1. i n t h i s q u e s t i o n , 999 , 1011 a r e o d d a n d t h e i r s u m i s = 2010 w h i c h i s 2011 1 t h e r e f o r e R s h o u l d b e 1. let\quad consider\quad x+y=p-1\quad where\quad p\quad is\quad prime,\\ when\quad x!\times y!\quad is\quad divided\quad by\quad p\quad here's\quad I\quad need\quad your\quad help,\\ it\quad is\quad quite\quad obvious\quad that\quad both\quad x\quad and\quad y\quad will\quad be\quad odd\quad simultaneously\quad or\quad even,\\ therefore\quad what\quad is\quad hypothesized\quad that\quad x,y\quad are\quad odd\quad remainder\quad is\quad 1\\ and\quad when\quad even\quad remainder\quad is\quad -1.\\ in\quad this\quad question,999,1011\quad are\quad odd\quad and\quad their\quad sum\quad is=2010\quad which\quad is\quad 2011-1\\ therefore\quad R\quad should\quad be\quad 1.\\ now its your your turn to prove my hypothesis is wrong,i have been writing prove for it! also comment if there's a theorem related to my hypothesis.

Please attach your proof when you are done.

Jatin Gahlawat - 7 years ago
Sabih Hasan
Jan 18, 2017

Write 1011! x 999! as 1006 x 2 x (1011 x 1010 x ... x 1007 x 1005 x ... x1).

Note that 1006 x 2 = 2012 = 1 (mod 2011). Since bracket is just a multiple of 2012, we conclude that 1011! x 999! = 1 (mod 2011).

Kevin Patel
Jun 6, 2014

Of course the solution of such Odd factorials would always be 1

@Kevin Patel, how about (7!*9!/11!) where the remainder is 9?

Ajit Athle - 6 years, 11 months ago

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