2014 AMC 12B Problem 23

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23.

The number 20172017 is prime. Let S=k=062(2014k).S = \sum_{k=0}^{62} \binom{2014}{k}. What is the remainder when SS is divided by 2017?2017?

3232

684684

10241024

15761576

20162016

Answer: C
Concepts:combinationsmodular arithmetictelescoping
Difficulty rating: 2560
Solution:

Working modulo 2017,2017, the identity (2014k)k!(2014k)!=2014!\binom{2014}{k} \cdot k! \cdot (2014-k)! = 2014! together with 20162015(2015k)2016 \cdot 2015 \cdots (2015-k) (1)k(k+2)!\equiv (-1)^k (k+2)! leads to 2(2014k)(1)k(k+2)(k+1)(mod2017), \begin{gathered} 2\binom{2014}{k} \equiv (-1)^k \\ {}\cdot (k+2)(k+1) \pmod{2017}, \end{gathered} so (2014k)(1)k(k+22).\binom{2014}{k} \equiv (-1)^k \binom{k+2}{2}.

Then Sk=062(1)k(k+22)=1+k=131[(2k+22)(2k+12)]=1+k=131(2k+1). \begin{gathered} S \equiv \sum_{k=0}^{62} (-1)^k \binom{k+2}{2} \\ = 1 \\ {}+ \sum_{k=1}^{31}\left[\binom{2k+2}{2} - \binom{2k+1}{2}\right] \\ = 1 + \sum_{k=1}^{31}(2k+1). \end{gathered}

The remaining sum is 3+5++63=1023,3 + 5 + \cdots + 63 = 1023, so S1+1023S \equiv 1 + 1023 =1024(mod2017).= 1024 \pmod{2017}.

Thus, the correct answer is C.

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