Push it to the limit

Level 2

The value of

lim n i = 1 2014 ( 1 + i n ) n \displaystyle\lim_{n \to \infty} \prod_{i=1}^{2014} (1+\frac{i}{n})^n

can be expressed as e x e^x for some number x.What are the last three digits of x?


The answer is 105.

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2 solutions

Jubayer Nirjhor
Jan 29, 2014

We'll use the well known identity lim n ( 1 + k n ) n = e k \lim_{n\rightarrow\infty} \left(1+\dfrac{k}{n}\right)^n=e^k Therefore lim n i = 1 2014 ( 1 + i n ) n = i = 1 2014 e i = e i = 1 2014 i = e 2029105 \lim_{n\rightarrow \infty} \prod_{i=1}^{2014}\left(1+\dfrac{i}{n}\right)^n=\prod_{i=1}^{2014}e^i=e^{\sum_{i=1}^{2014}i}=e^{2029105}

The desired answer is, therefore, 105 \fbox{105} .

Bogdan Simeonov
Jan 29, 2014

It is a well known fact that

lim n ( 1 + i n ) n = e i \displaystyle\lim_{n \to \infty} (1+\frac{i}{n})^n=e^i

for some constant i.That can be proved by using the Taylor series for e x e^x and expanding with the binomial formula and taking the limit as n approaches infinity.Then our product will be

e . e 2 . e 3 . . . . e 2014 = e 1007.2015 e.e^2.e^3....e^{2014}=e^{1007.2015} .Taking mod 1000 we get the answer 105 \boxed{105} .

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