Just Observe The Terms

Find the remainder when 1 ! + 2 ! + 3 ! + + 2016 ! 1! + 2!+ 3! +\cdots + 2016! is divided by 12.


The answer is 9.

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2 solutions

All numbers greater than 3 ! 3! are divisible by 12 12 . So remainder is 1 ! + 2 ! + 3 ! = 1 + 2 + 6 = 9 1!+2!+3!=1+2+6=\boxed9 .

You got it so easy! Nice +1

Viki Zeta - 4 years, 11 months ago
Viki Zeta
Jun 30, 2016

We know n ! = ( n ) × ( n 1 ) × ( n 2 ) × . . . × ( 2 ) × ( 1 ) That means, for n m , n ! is divisible by m, as n! contains m in it. For example, 3! = 3 × 2 × 1 = 6 , if m = 2, then 3! is divisible by 2, 3!/2 = 6/2 = 3 Using the above fact, we now gonna apply that in here. Taking n as 1 to 2016, and m as 12. All the factorials 12 ! is divisible by 12 and leaves the remainder "0". Therefore the remainder of 1! + 2! + ... + 2016! is when Sum-of n! divided by 12 for all n 11 = > 1 ! + 2 ! + 3 ! + 4 ! + . . . + 11 ! = 43954713 = > 1 ! + 2 ! + 3 ! + 4 ! + . . . + 11 ! = 12 3662892 + 9 \text{We know } n! = (n)\times(n-1)\times(n-2)\times ...\times (2)\times(1)\\ \text{That means, for }n \geq m, n! \text{ is divisible by m, as n! contains m in it.}\\ \text{For example, 3! = }3\times2\times1 = 6, \text{ if m = 2, then 3! is divisible by 2, 3!/2 = 6/2 = 3} \\ \text{Using the above fact, we now gonna apply that in here. Taking n as 1 to 2016, and m as 12. All the factorials } \geq 12! \\\text{is divisible by 12 and leaves the remainder "0".}\\ \text{Therefore the remainder of 1! + 2! + ... + 2016! is when Sum-of n! divided by 12 for all n } \leq \text{ 11}\\ => 1! + 2! + 3! + 4! + ... + 11! = 43954713 \\ =>1! + 2! + 3! + 4! + ... + 11! = 12 * 3662892 + 9

Therefore the remainder is 9.

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