Will you divide it?

What is the remainder obtained when 1 ! + 2 ! + 3 ! + + 200 ! 1! + 2! + 3! +\cdots+ 200! is divided by 14.

Notation : ! ! denotes the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .

1 5 7 13 None of These 11

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2 solutions

Chew-Seong Cheong
Jul 24, 2016

We note that n ! n! is divisible by 14 for n 7 n \ge 7 . Therefore, we have:

n = 1 200 n ! n = 1 6 n ! (mod 14) ( 1 ! + 2 ! + 3 ! + 4 ! + 5 ! + 6 ! ) (mod 14) Note that 5 ! + 6 ! = 7 5 ! which is divisible by 7 ( 1 ! + 2 ! + 3 ! + 4 ! + 0 ) (mod 14) ( 1 + 2 + 6 + 24 ) (mod 14) ( 1 + 2 + 6 + 10 ) (mod 14) 19 (mod 14) 5 (mod 14) \begin{aligned} \sum_{n=1}^{200} n! & \equiv \sum_{n=1}^6 n! \text{ (mod 14)} \\ & \equiv (1! + 2! + 3! + 4! + \color{#3D99F6}{5! + 6!}) \text{ (mod 14)} & \small \color{#3D99F6}{\text{Note that } 5!+6! = 7 \cdot 5! \text{ which is divisible by }7} \\ & \equiv (1! + 2! + 3! + 4! + \color{#3D99F6}{0}) \text{ (mod 14)} \\ & \equiv (1 + 2 + 6 + 24) \text{ (mod 14)} \\ & \equiv (1 + 2 + 6 + 10) \text{ (mod 14)} \\ & \equiv 19 \text{ (mod 14)} \\ & \equiv \boxed{5} \text{ (mod 14)} \end{aligned}

Satyabrata Dash
Jul 24, 2016

Hey this is really easy

1 ! = 1 1! =1

2 ! = 2 2! =2

3 ! = 6 3! =6

4 ! = 24 4!=24

But in and after 5 ! 5! the values are going to end with 0 0 due to 2 × 5

So it comes down to ( 1 ! + 2 ! + 3 ! + 4 ! ) ( m o d 14 ) (1!+2!+3!+4!)(mod 14) = 33 ( m o d 14 ) 33(mod14) = 5 \boxed{5}

How does ending with 0 relate to divisibility by 14?

Arulx Z - 4 years, 10 months ago

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