ln ( α = 1 ∑ ∞ ( ∫ 0 ∞ 2 0 1 8 [ ( α − 1 ) ! ] 2 x α − 1 e − 2 0 1 8 x d x ) )
Find the value of the expression above.
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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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I = ln ⎝ ⎛ α = 1 ∑ ∞ ( ∫ 0 ∞ 2 0 1 8 [ ( α − 1 ) ! ] 2 x α − 1 e − 2 0 1 8 x d x ) ⎠ ⎞
I = ln ⎝ ⎛ α = 1 ∑ ∞ 2 0 1 8 [ ( α − 1 ) ! ] 2 1 ( ∫ 0 ∞ x α − 1 e − 2 0 1 8 x d x ) ⎠ ⎞
Make x = 2 0 1 8 u , d x = 2 0 1 8 d u :
I = ln ⎝ ⎛ α = 1 ∑ ∞ 2 0 1 8 [ ( α − 1 ) ! ] 2 1 ( ∫ 0 ∞ ( 2 0 1 8 u ) α − 1 e − u 2 0 1 8 d u ) ⎠ ⎞
I = ln ⎝ ⎛ α = 1 ∑ ∞ [ ( α − 1 ) ! ] 2 2 0 1 8 α − 1 ( ∫ 0 ∞ u α − 1 e − u d u ) ⎠ ⎞
The integral is definition of the Gamma Function at α , which is equal to ( α − 1 ) !
I = ln ⎝ ⎛ α = 1 ∑ ∞ ( α − 1 ) ! 2 0 1 8 α − 1 ⎠ ⎞
Make n = α − 1 :
I = ln ⎝ ⎛ n = 0 ∑ ∞ n ! 2 0 1 8 n ⎠ ⎞
From Maclaurin series of exponential function:
I = ln ( e 2 0 1 8 )
I = 2 0 1 8