Another Logarithm Problem.(14)

Calculus Level 5

1 ( ln x x ) 2011 d x = b ! a c \large \displaystyle \int_1^\infty \left(\frac{\ln x}{x} \right)^{2011} dx = \frac{b!}{a^c}

If the above equation is true, where a a , b b , and c c are positive integers , find a + b + c a + b + c .


The answer is 6033.

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1 solution

By chain rule, d d x ( ln x ) n = n ln n 1 x x \large \displaystyle \text{By chain rule, } \frac{\text{d}}{\text{d}x} \left(\ln x \right)^{n} = \frac{n \ln^{n-1} x}{x}

By using integration By Parts \large \displaystyle \text{By using integration By Parts}

1 ( ln x ) 2011 x 2011 d x = [ ( ln x ) 2011 2010 x 2010 ] 1 1 n ( ln x ) n 1 2010 x 2011 d x \large \displaystyle \int_{1}^{\infty} \frac{(\ln x)^{2011}}{x^{2011}} \, dx = \left[\frac{(\ln x)^{2011}}{-2010 x^{2010}} \right]_1^{\infty} - \int_1^{\infty} \frac{n(\ln x)^{n-1}}{-2010x^{2011}} \, dx

ln ( 1 ) = 0 , lim x ( ln x ) n x 2010 = 0 n > 0. \large \displaystyle \ln(1) = 0, \lim_{x \rightarrow \infty} \frac{(\ln x)^{n}}{x^{2010}} = 0 \, \forall \, n > 0.

1 ( ln x ) 2011 x 2011 d x = 1 n ( ln x ) n 1 2010 x 2011 d x \large \displaystyle \int_1^{\infty} \frac{(\ln x)^{2011}}{x^{2011}} \, dx = \int_1^{\infty} \frac{n (\ln x)^{n-1}}{2010 x^{2011}} \, dx

1 ( ln x ) 2011 x 2011 d x = 2011 ! 201 0 2011 1 1 x 2011 d x = 2011 ! 2010 2012 \large \displaystyle \implies \int_1^{\infty} \frac{(\ln x)^{2011}}{x^{2011}} \, dx = \frac{2011!}{2010^{2011}} \int_1^{\infty} \frac{1}{x^{2011}} \, dx = \frac{\color{#D61F06}{2011!}}{\color{#20A900}{2010}^{\color{#3D99F6}{2012}}} .

b ! a c = 2011 ! 2010 2012 \huge \displaystyle \frac{\color{#D61F06}{b!}}{\color{#20A900}{a}^{\color{#3D99F6}{c}}} = \frac{\color{#D61F06}{2011!}}{\color{#20A900}{2010}^{\color{#3D99F6}{2012}}}

a + b + c = 2010 + 2011 + 2012 = 6033 \huge \displaystyle \color{#20A900}{a} + \color{#D61F06}{b} + \color{#3D99F6}{c} = \color{#20A900}{2010} + \color{#D61F06}{2011} + \color{#3D99F6}{2012} = \color{#EC7300}{\boxed{6033}}

Typo : Remove the extra zero in the final answer

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Yeah. Thank You!

Samara Simha Reddy - 5 years ago

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