That pesky 1

Calculus Level 4

0 π / 2 sin x + 1 sin x + cos x d x \large \displaystyle \int_{0}^{\pi/2} \dfrac{\sin x + 1}{\sin x + \cos x} \, dx

If the above integral can be expressed in the form π a + cos ( π b ) ln ( c + d c d ) , \dfrac{\pi}{a} + \cos \left( \dfrac{ \pi }{b}\right) \ln \left( \dfrac{\sqrt{c }+ d}{\sqrt{c} - d} \right) ,

where a , b , c a, b, c and d d are positive integers, with c c square-free, find a + b + c + d a + b + c + d .


The answer is 11.

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1 solution

Rishabh Jain
May 7, 2016

Call the integration E \mathfrak E and then apply a b f ( x ) d x = a b f ( a + b x ) d x \displaystyle\int_a^b f(x)\mathrm{d}x=\displaystyle\int_a^b f(a+b-x)\mathrm{d}x and add them to get:- E = 1 2 ( 0 π / 2 sin x + cos x + 2 sin x + cos x d x ) = π 4 + 0 π / 2 d x sin x + cos x R R = 0 π / 2 d x cos ( π / 2 x ) + cos x 2 cos ( π / 4 ) cos ( π / 4 x ) = ( 1 / 2 ) ( 0 π / 2 sec ( π / 4 x ) \ d x ) = ( 1 / 2 ) [ ln ( sec ( π / 4 x ) + tan ( π / 4 x ) ) ] 0 π / 2 = ( 1 / 2 cos ( π / 4 ) ) ln ( ( 2 + 1 2 1 ) ) \begin{aligned}\mathfrak{E}=&\dfrac 12\left(\large \displaystyle \int_{0}^{\pi/2} \dfrac{\color{teal}{\sin x +\cos x}+ 2}{\color{teal}{\sin x + \cos x}} \, \mathrm{d}x\right)\\=&\dfrac{\pi}{4}+\underbrace{\displaystyle\int_0^{\pi/2}\dfrac{\mathrm{d}x}{\sin x+\cos x}}_{\mathfrak R}\\\mathfrak R=&\displaystyle\int_0^{\pi/2}\dfrac{\mathrm{d}x}{\underbrace{\cos\left(\pi/2 -x\right)+\cos x}_{2\cos (\pi/4)\cos (\pi/4-x)}}\\=&(1/\sqrt{2})\left(\displaystyle\int_0^{\pi/2}\sec(\pi/4-x)\ \mathrm{d}x\right)\\=& (-1/\sqrt 2)\left[\ln(\sec (\pi/4-x)+\tan (\pi/4-x))\right]_0^{\pi/2}\\=&(\underbrace{1/\sqrt 2}_{\color{#D61F06}{\cos (\pi/4)}})\ln\left(\left( \dfrac{\sqrt 2+1}{\sqrt 2-1}\right)\right)\end{aligned}

Hence, combining we get;-

E = π / 4 + cos ( π 4 ) ln ( 2 + 1 2 1 ) \large \mathfrak E=\pi/4+\cos\left(\dfrac{\pi}{4}\right)\ln\left(\dfrac{\sqrt 2+1}{\sqrt 2-1}\right)

4 + 4 + 2 + 1 = 11 \large\therefore ~4+4+2+1=\Huge\boxed{\color{#20A900}{11}}

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