Don't be intimidated!

Calculus Level pending

0 ln 2 2 x 3 e x 2 e x 2 1 d x = a π b c \int_0^{\sqrt{\ln2}}\frac{2x^3e^{x^2}}{e^{x^2}-1}dx=\frac{a\pi^b}{c}

The equation above holds true for positive integers a a , b b , and c c , where a a and c c are coprime. Find the value of a b c abc .


The answer is 24.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Aaghaz Mahajan
Jun 18, 2019

Substituting e x 2 1 = t \displaystyle e^{x^2}-1=t the integral changes to

0 1 ln ( 1 + t ) t d t \int_0^1\frac{\ln\left(1+t\right)}{t}dt

= 0 1 n = 1 ( t ) n 1 n d t =\int_0^1\sum_{n=1}^{\infty}\frac{\left(-t\right)^{n-1}}{n}dt

= n = 1 ( 0 1 ( t ) n 1 n d t ) =\sum_{n=1}^{\infty}\left(\int_0^1\frac{\left(-t\right)^{n-1}}{n}dt\right)

= n = 1 ( 1 ) n 1 n 2 =\sum_{n=1}^{\infty}\frac{\left(-1\right)^{n-1}}{n^2}

= ( n = 1 1 n 2 ) ( 1 2 n = 1 1 n 2 ) =\left(\sum_{n=1}^{\infty}\frac{1}{n^2}\right)-\left(\frac{1}{2}\sum_{n=1}^{\infty}\frac{1}{n^2}\right)

= π 2 12 =\frac{\pi^2}{12}

Thus, making the answer 1 2 12 = 24 \displaystyle 1\cdot2\cdot12=24

Note

n = 1 1 n 2 = π 2 6 \displaystyle \sum_{n=1}^{\infty}\frac{1}{n^2}=\frac{\pi^2}{6} follows from the Riemann Zeta Function

Nicely written. Thank you.

Pi Han Goh - 1 year, 11 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...