A Definite Problem

Calculus Level 4

ln 2 ln 3 x sin ( x 2 ) sin ( x 2 ) + sin ( ln 6 x 2 ) d x \large \int_{\sqrt{\ln 2}}^{\sqrt{ \ln 3}} \dfrac {x \sin (x^2)}{\sin(x^2) + \sin(\ln 6 - x^2)} \, dx

The integral above has a closed form. Find the value of this closed form.

Give your answer to 3 decimal places.


The answer is 0.101.

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2 solutions

Rishabh Jain
Jul 12, 2016

Relevant wiki: Integration Tricks

Substitute x 2 = t x^2=t such that 2 x d x = d t 2x\mathrm{d}x=\mathrm{d}t

I = 1 2 ln 2 ln 3 sin t d t sin t + sin ( ln 6 t ) \color{#3D99F6}{\mathcal I}=\dfrac 12\int_{\ln 2}^{\ln 3}\dfrac{\sin t~\mathrm{d}t}{\sin t+\sin (\ln 6-t)}

Now use a b f ( x ) d x = a b f ( a + b x ) d x \small{\color{teal}{\displaystyle\int_a^bf(x)\mathrm{d}x=\displaystyle\int_a^b f(a+b-x)\mathrm{d}x}} so that:

I = 1 2 ln 2 ln 3 sin ( ln 6 t ) d t sin t + sin ( ln 6 t ) \color{#3D99F6}{\mathcal I}=\dfrac 12\int_{\ln 2}^{\ln 3}\dfrac{\sin (\ln 6-t)~\mathrm{d}t}{\sin t+\sin (\ln 6-t)}

Adding we get I = 1 4 ln 2 ln 3 d t = 1 4 ln ( 3 2 ) 0.101 \mathcal I=\dfrac 14\int_{\ln 2}^{\ln 3}\mathrm{d}t=\dfrac 14\ln \left(\dfrac 32\right)\approx \boxed{0.101}

Same solution again, we are so in sync

Michael Fuller - 4 years, 11 months ago

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Haha... I have this similarity with many people on brilliant.... Welcome to that club. XD

Rishabh Jain - 4 years, 11 months ago

Did the same way. (+1)

Ashish Menon - 4 years, 11 months ago

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Great.....

Rishabh Jain - 4 years, 11 months ago
Joe Potillor
Feb 1, 2017

I did it the long way, but that's alright, someone ought to show the long way too :p

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