Another Typical integral !!!

Calculus Level 4

ln ( α = 1 ( 0 x α 1 e x 2018 2018 [ ( α 1 ) ! ] 2 d x ) ) \ln \left(\sum_{\alpha=1}^{\infty}\left(\int_{0}^{\infty} \frac{x^{\alpha-1} e^{-\frac{x}{2018}}}{2018[(\alpha-1) !]^{2}} d x\right)\right)

Find the value of the expression above.


The answer is 2018.

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2 solutions

Guilherme Niedu
May 6, 2019

I = ln ( α = 1 ( 0 x α 1 e x 2018 2018 [ ( α 1 ) ! ] 2 d x ) ) \large \displaystyle I = \ln \left ( \sum_{\alpha = 1}^{\infty} \left ( \int_0^{\infty} \frac{x^{\alpha - 1} e^{- \frac{x}{2018}}}{2018[(\alpha-1)!]^2} dx \right ) \right )

I = ln ( α = 1 1 2018 [ ( α 1 ) ! ] 2 ( 0 x α 1 e x 2018 d x ) ) \large \displaystyle I = \ln \left ( \sum_{\alpha = 1}^{\infty} \frac{1}{2018[(\alpha-1)!]^2} \left ( \int_0^{\infty} x^{\alpha - 1} e^{- \frac{x}{2018}} dx \right ) \right )

Make x = 2018 u x = 2018u , d x = 2018 d u dx = 2018 du :

I = ln ( α = 1 1 2018 [ ( α 1 ) ! ] 2 ( 0 ( 2018 u ) α 1 e u 2018 d u ) ) \large \displaystyle I = \ln \left ( \sum_{\alpha = 1}^{\infty} \frac{1}{2018[(\alpha-1)!]^2} \left ( \int_0^{\infty} (2018u)^{\alpha - 1} e^{-u} 2018 du \right ) \right )

I = ln ( α = 1 201 8 α 1 [ ( α 1 ) ! ] 2 ( 0 u α 1 e u d u ) ) \large \displaystyle I = \ln \left ( \sum_{\alpha = 1}^{\infty} \frac{2018^{\alpha - 1}}{[(\alpha-1)!]^2} \left ( \int_0^{\infty} u^{\alpha - 1} e^{-u} du \right ) \right )

The integral is definition of the Gamma Function at α \alpha , which is equal to ( α 1 ) ! (\alpha-1)!

I = ln ( α = 1 201 8 α 1 ( α 1 ) ! ) \large \displaystyle I = \ln \left ( \sum_{\alpha = 1}^{\infty} \frac{2018^{\alpha - 1}}{( \alpha-1)!} \right )

Make n = α 1 n = \alpha - 1 :

I = ln ( n = 0 201 8 n n ! ) \large \displaystyle I = \ln \left ( \sum_{n = 0}^{\infty} \frac{2018^n}{n!} \right )

From Maclaurin series of exponential function:

I = ln ( e 2018 ) \large \displaystyle I = \ln \left ( e^{2018} \right )

I = 2018 \color{#3D99F6} \boxed { \large \displaystyle I = 2018 }

After integrating, the given expression becomes the logarithm of summation of (2018)^(α-1)/[(α-1)!] from α=1 to infinity, or the logarithm of exp(2018), which is equal to 2018.

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