A 2015th degree polynomial satisfies the equation for . Find the remainder when is divided by 7.
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Let g(x)=x(x-1)(x-2)....(x-n). Thus f(x) can be assumed as ∑ k = 0 n a k 2 k g(x)/(x-k). Now we can choose the values of f(k) with the help of companion function g(x). Now obviously we will choose a k in a way that f(k)= 2 k . It's not difficult to see that a k = ( − 1 ) k + 1 2015Ck/2015! (where nCr is the coefficient of x r in expansion of ( 1 + x ) n ). Now put x=n+1 in f(x), it becomes ∑ k = 0 n ( − 1 ) k + 1 (n+1)Ck 2 k which simplifies to 2 n + 1 -1. Put n=2015, and you have f(2016)= 2 2 0 1 6 -1= 8 6 7 2 -1=(8-1)*( 8 6 7 1 + 8 6 7 0 +...+8²+8+1), clearly a multiple of 7.
This is my first explanation using latex :)