A Question on Polynomials, part 2

Algebra Level 5

Given that f f is a polynomial of degree 100 and that f ( k ) = 2 k f(k)=2^k for k = 1 , 2 , , 101 k=1, 2, \ldots , 101 . Find the last five digits of f ( 102 ) f(102) .

You may use the fact that the llast five digits of 2 101 2^{101} is 10752.


The answer is 21502.

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1 solution

Chan Lye Lee
Mar 31, 2016

Let f ( x ) = 2 ( ( x 1 0 ) + ( x 1 1 ) + + ( x 1 100 ) ) f(x)= 2\left({x-1 \choose 0} +{x-1 \choose 1}+\ldots +{x-1 \choose 100}\right)

We see that this the degree of f f is 100 and it fulfills and the condition f ( k ) = 2 k f(k)=2^k for k = 1 , 2 , , 101 k=1, 2, \ldots , 101 .

Now f ( 102 ) = 2 ( ( 101 0 ) + ( 101 1 ) + + ( 101 100 ) ) = 2 ( 2 101 1 ) = 2 102 2 f(102)= 2\left({101 \choose 0} +{101 \choose 1}+\ldots +{101 \choose 100}\right) =2\left(2^{101}-1 \right) = 2^{102}-2 . Since it is given that the last five digits of 2 101 2^{101} is 10752, then the last five digits of 2 102 2 2^{102}-2 is 21502.

Note : the question is modified from a question in Pi Mu Epsilon , (1964).

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