Consider F ( a ) = ∫ 0 1 ln a ( x ) d x . Find ln ( a = 0 ∑ ∞ F ( a ) 1 ) .
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consider ∫ 0 1 x n dx = n + 1 1 d.w.r.n a time and putting n = 0 to get ∫ 0 1 ln a ( x ) dx = ( − 1 ) a a ! now put this in the summation a = 0 ∑ ∞ a ! ( − 1 ) a = e − 1 In the last step the taylor series of e x was used.
Can you explain how we "d.w.r.n a time and putting n = 0"?
Can you explain how we "d.w.r.n a time and putting n = 0"?
I was gonna write a solution, but you already wrote the exact method I used
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By Integration by Parts: F ( a ) = x ln a x ∣ 0 1 − a ∫ 0 1 ln a − 1 ( x ) d x = − a F ( a − 1 ) ( ∵ x ln a x ∣ 0 1 = 0 ) = a ( a − 1 ) F ( a − 2 ) = − a ( a − 1 ) ( a − 2 ) F ( a − 3 ) ⋯ ⋯ = ( − 1 ) a a ! F ( 0 ) = ( − 1 ) a a ! ( ∵ F ( 0 ) = 1 ) ∴ ln ⎝ ⎛ a = 0 ∑ ∞ F ( a ) 1 ⎠ ⎞ = ln ( e 1 ) = − 1
N O T E : Consider expansion of e x : e x = 1 + 1 ! x + 2 ! x 2 + 3 ! x 3 ⋯ Putting x=-1, we get e 1 = a = 0 ∑ ∞ ( − 1 ) a a ! 1 = a = 0 ∑ ∞ F ( a ) 1