Floor of a number difficult to compute

Find ( 1 0 2018 ) ! 3 ( 5 1 0 2018 ) 1 0 2017 1 0 2018 + 1009 \Bigl\lfloor \frac{(10^{2018})!}{3(5^{10^{2018}})10^{2017*10^{2018}+1009}}\Bigr\rfloor


The answer is 0.

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1 solution

Arturo Presa
Jun 16, 2019

In the inequality n ! e n n + 1 2 e n n! \leq e\;n^{n+\frac{1}{2}} e^{-n} (see the Stirling's approximation ), make n = 1 0 2018 , n=10^{2018}, and use the fact that 2 < e < 3. 2<e<3. Then you get ( 1 0 2018 ) ! e ( 1 0 2018 ) ( 1 0 2018 + 1 2 ) ( e 1 0 2018 ) < 3 ( 1 0 2018 2 ) 1 0 2018 1 0 1009 = 3 ( 5 1 0 2018 ) 1 0 2017 1 0 2018 + 1009 . (10^{2018})!\leq e (10^{2018})^{(10^{2018}+\frac{1}{2})} (e^{-10^{2018}})< 3*(\frac{10^{2018}}{2})^{10^{2018}} 10^{1009}=3(5^{10^{2018}}) 10^{2017*10^{2018}+1009}. Dividing both sides by the number in the right side of the inequality, ( 1 0 2018 ) ! 3 ( 5 1 0 2018 ) 1 0 2017 1 0 2018 + 1009 < 1. \frac{(10^{2018})!}{3(5^{10^{2018}}) 10^{2017*10^{2018}+1009}}<1. Therefore, ( 1 0 2018 ) ! 3 ( 5 1 0 2018 ) 1 0 2017 1 0 2018 + 1009 = 0 \Bigl\lfloor \frac{(10^{2018})!}{3(5^{10^{2018}}) 10^{2017*10^{2018}+1009}}\Bigr\rfloor=\boxed{0}

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