Jee Main 2018 # 2

Calculus Level 3

lim n r = 0 n ( n r ) n r ( r + 3 ) = e q \large \lim_{n \to \infty} \sum_{r=0}^n \frac {\dbinom nr}{n^r(r+3)} = e-q

The equation above holds true for positive integer q q . Find q q q^q .

Notation: e 2.718 e\approx 2.718 denotes the Euler's number .

Source: Fiitjee AITS


The answer is 4.

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2 solutions

Hassan Abdulla
Apr 8, 2018

L = lim n r = 0 n ( n r ) n r ( r + 3 ) L = lim n r = 0 n ( ( n r ) n r 0 1 x r + 2 d x ) L = lim n 0 1 ( x 2 r = 0 n ( n r ) x r n r ) d x L = lim n 0 1 ( x 2 r = 0 n ( n r ) ( x n ) r 1 n r ) d x L = lim n 0 1 ( x 2 ( 1 + x n ) n ) d x L = 0 1 ( x 2 lim n ( 1 + x n ) n ) d x L = 0 1 ( x 2 e x ) d x = ( x 2 2 x + 2 ) e x 0 1 = e 2 s o q = 2 a n d q q = 4 L=\lim _{ n\to \infty } \sum _{ r=0 }^{ n }{ \frac { \begin{pmatrix} n \\ r \end{pmatrix} }{ n^{ r }(r+3) } } \\ \\ L=\lim _{ n\to \infty } \sum _{ r=0 }^{ n }{ \left( \frac { \begin{pmatrix} n \\ r \end{pmatrix} }{ n^{ r } } \int _{ 0 }^{ 1 }{ x^{ r+2 }dx } \right) } \\ \\ L=\lim _{ n\to \infty } \int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }\sum _{ r=0 }^{ n }{ \begin{pmatrix} n \\ r \end{pmatrix}\frac { x^{ r } }{ n^{ r } } } \right) dx } \\ \\ L=\lim _{ n\to \infty } \int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }\sum _{ r=0 }^{ n }{ \begin{pmatrix} n \\ r \end{pmatrix}{ \left( \frac { x }{ n } \right) }^{ r }{ 1 }^{ n-r } } \right) dx } \\ \\ L=\lim _{ n\to \infty } \int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }\left( 1+\frac { x }{ n } \right) ^{ n } \right) dx } \\ \\ L=\int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }\lim _{ n\to \infty } \left( 1+\frac { x }{ n } \right) ^{ n } \right) dx } \\ \\ L=\int _{ 0 }^{ 1 }{ \left( { x }^{ 2 }{ e }^{ x } \right) dx } =\left. \left( x^{ 2 }-2x+2 \right) { e }^{ x } \right\vert _{ 0 }^{ 1 }=e-2\\ \\ so\quad q=2\quad and\quad { q }^{ q }=4

limit value equal to e-2 therefore q = 2

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