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Calculus Level 3

0 π / 2 l o g ( 4 + 3 s i n x 4 + 3 c o s x ) d x = \int _{ 0 }^{ \pi /2 }{ \quad log\begin{pmatrix} \cfrac { 4\quad +\quad 3\quad sin\quad x }{ 4\quad +\quad 3\quad cos\quad x } & \\ & \end{pmatrix} } dx\quad =


The answer is 0.

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2 solutions

Samarth Sangam
Aug 21, 2014

L e t I n = 0 π / 2 log 4 + 3 sin x 4 + 3 cos x d x t h e n b y b a f ( x ) d x = b a f ( a + b x ) d x w e g e t I n = 0 π / 2 log 4 + 3 sin π / 2 x 4 + 3 cos π / 2 x d x a n d c a n b e w r i t t e n a s I n = 0 π / 2 log 4 + 3 cos x 4 + 3 sin x d x a n d 2 I n = I n + I n = 0 π / 2 log 4 + 3 cos x 4 + 3 sin x d x + 0 π / 2 log 4 + 3 sin x 4 + 3 cos x d x = 0 π / 2 ( log 4 + 3 cos x 4 + 3 sin x log 4 + 3 cos x 4 + 3 sin x ) d x = 0 π / 2 0 d x = 0 Let\quad { I }_{ n }=\int _{ 0 }^{ \pi /2 }{ \log { \frac { 4+3\sin { x } }{ 4+3\cos { x } } } } dx\\ then\quad by\quad \int _{ b }^{ a }{ f\left( x \right) dx } =\int _{ b }^{ a }{ f\left( a+b-x \right) dx } \\ we\quad get\quad { I }_{ n }=\int _{ 0 }^{ \pi /2 }{ \log { \frac { 4+3\sin { \pi /2-x } }{ 4+3\cos { \pi /2-x } } } } dx\\ and\quad can\quad be\quad written\quad as\quad { I }_{ n }=\int _{ 0 }^{ \pi /2 }{ \log { \frac { 4+3\cos { x } }{ 4+3\sin { x } } } } dx\\ and\quad 2{ I }_{ n }={ I }_{ n }+{ I }_{ n }\\ \quad \quad \quad \quad \quad \quad \quad =\int _{ 0 }^{ \pi /2 }{ \log { \frac { 4+3\cos { x } }{ 4+3\sin { x } } } } dx+\int _{ 0 }^{ \pi /2 }{ \log { \frac { 4+3\sin { x } }{ 4+3\cos { x } } } } dx\\ \quad \quad \quad \quad \quad \quad \quad \quad =\int _{ 0 }^{ \pi /2 }{ (\log { \frac { 4+3\cos { x } }{ 4+3\sin { x } } } -\log { \frac { 4+3\cos { x } }{ 4+3\sin { x } } ) } dx } \\ \quad \quad \quad \quad \quad \quad \quad \quad =\int _{ 0 }^{ \pi /2 }{ 0 } dx\\ \quad \quad \quad \quad \quad \quad \quad \quad =0

Nice solution dude.

A Former Brilliant Member - 6 years, 9 months ago

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thank you for the comment

samarth sangam - 6 years, 9 months ago

even i like prince of persia

Vighnesh Raut - 6 years, 8 months ago

it is NCERT miscellaneous question ?? right !!

Rishabh Jain - 6 years, 9 months ago

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sorry I don't know

samarth sangam - 6 years, 9 months ago
Arjen Vreugdenhil
May 13, 2017

Consider the symmetry: The integrand may be written as I ( x ) = log ( 4 + 3 sin x ) log ( 4 + 3 cos x ) . I(x) = \log(4 + 3\sin x) - \log(4 + 3\cos x). This function is symmetric under mirroring x = π / 2 x x' =\pi/2 - x : I ( x ) = I ( π / 2 x ) = log ( 4 + 3 cos x ) log ( 4 + 3 sin x ) = I ( x ) . I(x') = I(\pi/2 - x) = \log(4 + 3\cos x) - \log(4 + 3\sin x) = -I(x). Thus we may split the integral as 0 π / 4 I ( x ) d x + π / 4 π / 2 I ( x ) d x = 0 π / 4 I ( x ) d x 0 π / 4 I ( x ) d x = 0 π / 4 ( I ( x ) I ( x ) ) d x = 0 . \int_0^{\pi/4} I(x)\ dx + \int_{\pi/4}^{\pi/2} I(x)\ dx \\ = \int_0^{\pi/4} I(x)\ dx - \int_0^{\pi/4} I(x')\ dx' \\ = \int_0^{\pi/4} (I(x) - I(x))\ dx = \boxed{0}.

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