Trigo-Fractional Integral

Calculus Level 5

0 π 2 100 { tan ( x ) } { cot ( x ) } d x \large \int_0^\frac \pi 2 100 \{\tan(x)\}\{\cot(x)\} \, dx

Find the value of the closed form of the above integral. Round off your answer to the nearest integer.


Notation: { } \{ \cdot \} denotes the fractional part function .


The answer is 27.

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

Mark Hennings
Jul 18, 2017

Note that f ( x ) = 100 { tan x } { cot x } f(x) = 100 \{\tan x\} \{\cot x\} is symmetric over [ 0 , 1 2 π ] [0,\tfrac12\pi] , and hence I = 0 1 2 π f ( x ) d x = 2 1 4 π 1 2 π f ( x ) d x = 200 1 4 π 1 2 π { tan x } cot x d x = 200 1 4 π 1 2 π ( tan x tan x ) cot x d x = 50 π 200 1 4 π 1 2 π tan x cot x d x = 50 π 200 n = 1 n tan 1 n tan 1 ( n + 1 ) cot x d x = 50 π 200 n = 1 n [ ln ( sin x ) ] tan 1 n tan 1 ( n + 1 ) = 50 π 200 lim N n = 1 N n [ ln ( n + 1 1 + ( n + 1 ) 2 ) ln ( n 1 + n 2 ) ] = 50 π 200 lim N { n = 2 N + 1 ( n 1 ) ln ( n 1 + n 2 ) n = 1 N n ln ( n 1 + n 2 ) } = 50 π 200 lim N { N ln ( N + 1 1 + ( N + 1 ) 2 ) n = 1 N ln ( 1 1 + n 2 ) } = 50 π + 100 lim N N ln ( 1 + ( N + 1 ) 2 ) 100 lim N ln ( n = 1 N ( 1 + n 2 ) ) = 50 π 100 ln ( n = 1 ( 1 + n 2 ) ) = 50 π 100 ln ( sinh π π ) = 26.89499282... \begin{aligned} I & = \int_0^{\frac12\pi}f(x)\,dx \; = \; 2\int_{\frac14\pi}^{\frac12\pi} f(x)\,dx \; = \; 200\int_{\frac14\pi}^{\frac12\pi} \{\tan x\} \cot x\,dx \\ & = 200\int_{\frac14\pi}^{\frac12\pi} \big(\tan x - \lfloor \tan x \rfloor \big) \cot x\,dx \; = \; 50\pi - 200\int_{\frac14\pi}^{\frac12\pi} \lfloor \tan x \rfloor \cot x\,dx \\ & = 50\pi - 200 \sum_{n=1}^\infty n \int_{\tan^{-1}n}^{\tan^{-1}(n+1)} \cot x\,dx \; = \; 50\pi - 200 \sum_{n=1}^\infty n \Big[\ln(\sin x) \Big]_{\tan^{-1}n}^{\tan^{-1}(n+1)} \\ & = 50\pi - 200 \lim_{N \to \infty}\sum_{n=1}^N n \left[ \ln\left(\frac{n+1}{\sqrt{1+(n+1)^2}}\right) - \ln\left(\frac{n}{\sqrt{1+n^2}}\right)\right] \\ & = 50\pi - 200\lim_{N \to \infty} \left\{ \sum_{n=2}^{N+1} (n-1)\ln\left(\frac{n}{\sqrt{1+n^2}}\right) - \sum_{n=1}^N n \ln\left(\frac{n}{\sqrt{1+n^2}}\right)\right\} \\ & = 50\pi - 200\lim_{N \to \infty}\left\{ N\ln\left(\frac{N+1}{\sqrt{1 + (N+1)^2}}\right) - \sum_{n=1}^N \ln\left(\frac{1}{\sqrt{1+n^{-2}}}\right)\right\} \\ & = 50\pi + 100 \lim_{N \to \infty} N \ln\left(1 + (N+1)^{-2}\right) - 100\lim_{N \to \infty}\ln\left(\prod_{n=1}^N\left(1 + n^{-2}\right)\right) \\ & = 50\pi - 100\ln\left(\prod_{n=1}^\infty \left(1 + n^{-2}\right)\right) \; =\; 50\pi - 100\ln\left(\frac{\sinh \pi}{\pi}\right) \; = \;26.89499282... \end{aligned} making the answer 27 \boxed{27} .

How did you solve last step i.e. getting sinhπ term?

Chaitnya Shrivastava - 3 years, 6 months ago

Log in to reply

It is a well-known fact that sin π z π z = n = 1 ( 1 z 2 n 2 ) \frac{\sin \pi z}{\pi z} \; = \; \prod_{n=1}^\infty \left(1 - \frac{z^2}{n^2}\right) a fact that is most easily proved by contour integration. Now put z = i z=i into this formula.

Mark Hennings - 3 years, 6 months ago

Log in to reply

Thank you.BTW is there any special name to it

Chaitnya Shrivastava - 3 years, 6 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...