That's too much of work

Calculus Level 5

0 π / 4 \displaystyle\int_0^{\pi/4} ln ( cot θ ) ( ( sin θ ) 2009 + ( cos θ ) 2009 ) 2 \dfrac{\ln(\cot \theta)}{\left((\sin \theta)^{2009}+(\cos \theta)^{2009}\right)^2} ( sin 2 θ ) 2008 (\sin 2\theta)^{2008} d θ d \theta = a b ln a c a \dfrac{a^b \ln a}{c^a}

If the equation above is true, solve the for values of a , b , c a,b,c where a , b , c a,b,c are in their simplest form. Find the value of a + b + c a+b+c .


The answer is 4019.

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2 solutions

Aman Rajput
Jun 16, 2015

Seriously i didn't know how to solve this, but i used this crazy fact :

Ignore log cot x \log\cot x ,because if i am going to use by-parts rule of integration i have to first calculate indefinite integral :

sin n 1 ( 2 x ) ( sin n x + cos n x ) 2 d x \displaystyle\int \frac{\sin^{n-1}(2x)}{(\sin^nx + \cos^nx)^2} dx

which on evaluating i get something of the form 2 n 1 n 2 g ( x , n ) + c \frac{2^{n-1}}{n^2}g(x,n) + c

For n = 2009 n=2009 it will be 2 2008 200 9 2 g ( x , 2009 ) + c \displaystyle \frac{2^{2008}}{2009^2}g(x,2009) + c As per required answer is in the form of a b log ( a ) c a \frac{a^b \log(a)}{c^a}

On comparing i thought it must be as follows : a = 2 , b = 2008 , c = 2009 a=2 , b=2008 , c=2009

a + b + c = 4019 a+b+c=4019

Please Dont laugh..!!! hahahahah!

Mahimn Bhatt
Mar 4, 2015

Open up sin(2theta)

Then divide the expression by (tan(theta) )^2009

Express the integral in the form of log(tan(theta)) and (tan(theta))^2009

Then put (tan(theta))^2009 = t

The expression will be simplified . Integrate the expression using by parts., to get a=2, b=2008, c=2009.

@Parth Lohomi its an amazing question. U framed it by urself???

Aditya Kumar - 5 years, 12 months ago

Nice problem @Parth Lohomi ! Took me 3 goes to get it right!

User 123 - 5 years, 12 months ago

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The problem was easy. But those 2009 2009 s made it a hell lot harder(A little exaggerated). Are you on G+?

Kartik Sharma - 5 years, 12 months ago

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Exactly! Those 2009 2009 nearly did me in. I have a Gmail account, but then I never use it.

User 123 - 5 years, 12 months ago

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@User 123 Oh! So, you're really a big nerd.

Kartik Sharma - 5 years, 12 months ago

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@Kartik Sharma This is actually wistful thinking, but I wish we could have something like this again. It would really help us all improve and learn some more Integration. Unfortunately, I doubt if anybody would have the time or interest in this anymore...

User 123 - 5 years, 12 months ago

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