Reciprocal logarithm integration

Calculus Level 3

1 0 x 2 + 2 x ln ( x + 1 ) d x = ? \large \int_{-1}^0 \dfrac{x^2+2x}{\ln(x+1)} \, dx = \, ?

ln 4 \ln4 ln 3 \ln3 ln 2 \ln2 ln 6 \ln6

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1 solution

Hassan Abdulla
Aug 20, 2020

I = 1 0 x 2 + 2 x ln ( x + 1 ) d x = 1 0 x 2 + 2 x + 1 1 ln ( x + 1 ) d x = 1 0 ( x + 1 ) 2 1 ln ( x + 1 ) d x I=\int_{-1}^0 \frac{x^2 + 2x}{\ln(x+1)}dx = \int_{-1}^0 \frac{x^2 + 2x {\color{#D61F06} + 1 - 1}}{\ln(x+1)}dx = \int_{-1}^0 \frac{(x+1)^2 - 1}{\ln(x+1)}dx\\

suppose that I ( a ) = 1 0 ( x + 1 ) a 1 ln ( x + 1 ) d x I(a) = \int_{-1}^0 \frac{(x+1)^a - 1}{\ln(x+1)}dx

our goal is I ( 2 ) I(2)

I ( a ) = 1 0 ( x + 1 ) a 1 ln ( x + 1 ) d x d d a I ( a ) = 1 0 ( x + 1 ) a d x = ( x + 1 ) a + 1 a + 1 1 0 = 1 a + 1 I ( a ) = ln ( a + 1 ) + C I ( 0 ) = 1 0 ( x + 1 ) 0 1 ln ( x + 1 ) d x = 0 I ( 0 ) = ln ( 0 + 1 ) + C C = 0 I ( a ) = ln ( a + 1 ) I ( 2 ) = ln ( 2 + 1 ) = ln ( 3 ) \begin{aligned} I(a) & = \int_{-1}^0 \frac{(x+1)^a - 1}{\ln(x+1)}dx\\ \frac{d}{da} I(a)& = \int_{-1}^0 (x+1)^a \ dx = \left . \frac{(x+1)^{a+1}}{a+1} \right |_{-1}^0 =\frac{1}{a+1} \\ I(a)& = \ln(a+1) + C \\ &{\color{#D61F06} I(0) =\int_{-1}^0 \frac{(x+1)^0 - 1}{\ln(x+1)}dx = 0 } \\ I(0)& = \ln(0+1) + C \Rightarrow C = 0 \\ I(a)& = \ln(a+1) \\ I(2)& = \ln(2+1) =\ln(3) \\ \end{aligned}

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