Definitely Violent

Calculus Level 5

0 π 2 x sin x d x + 0 π 2 ln ( 1 + tan x ) d x 0 π 4 ln ( ( 1 + tan x ) 2 ) d x 0 π 4 ln tan x d x 0 π 2 x tan x d x + 0 π 2 ln sin x d x + π 4 π 2 ln tan x d x = ? \frac{\displaystyle \int_{0}^{\frac{\pi}{2}}\frac{x}{\sin{x}}dx+\int_{0}^{\frac{\pi}{2}}\ln(1+\tan{x})\ dx-\int_{0}^{\frac{\pi}{4}}\ln((1+\tan{x})^2)dx-\int_{0}^{\frac{\pi}{4}}\ln\tan{x} \ dx}{\displaystyle \int_{0}^{\frac{\pi}{2}}\frac{x}{\tan{x}}dx+\int_{0}^{\frac{\pi}{2}}\ln\sin{x} \ dx+\int_{\frac{\pi}{4}}^{\frac{\pi}{2}}\ln\tan{x} \ dx}=?


The answer is 4.

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1 solution

Let I = 0 π 2 x sin x d x + 0 π 2 ln ( 1 + tan x ) d x 0 π 4 ln ( ( 1 + tan x ) 2 ) d x 0 π 4 ln ( tan x ) d x 0 π 2 x tan x d x + 0 π 2 ln ( sin x ) d x + π 4 π 2 ln ( tan x ) d x = I 1 + I 2 I 3 I 4 I 5 + I 6 + I 7 \displaystyle I = \frac {\displaystyle \int_0^\frac \pi 2 \frac x{\sin x}dx + \int_0^\frac \pi 2 \ln(1+\tan x)\ dx - \int_0^\frac \pi 4 \ln \left((1+\tan x)^2\right) dx - \int_0^\frac \pi 4 \ln(\tan x)\ dx}{\displaystyle \int_0^\frac \pi 2 \frac x{\tan x}dx + \int_0^\frac \pi 2 \ln(\sin x)\ dx + \int_\frac \pi 4^\frac \pi 2 \ln(\tan x)\ dx } = \frac {I_1+I_2-I_3-I_4}{I_5+I_6+I_7}

I 3 = 0 π 4 ln ( ( 1 + tan x ) 2 ) d x = 2 0 π 4 ln ( 1 + tan x ) d x \begin{aligned} I_3 & = \int_0^\frac \pi 4 \ln \left((1+\tan x)^2\right) dx = 2 \int_0^\frac \pi 4 \ln (1+\tan x)\ dx \end{aligned}

I 2 = 0 π 2 ln ( 1 + tan x ) d x = π 4 π 2 ln ( 1 + tan x ) d x + 0 π 4 ln ( 1 + tan x ) d x Using identity: a b f ( x ) d x = a b f ( a + b x ) d x = π 4 π 2 ln ( 1 + tan ( 3 π 4 x ) ) d x + 1 2 I 3 Note that tan ( π θ ) = tan θ = π 4 π 2 ln ( 1 tan ( x + π 4 ) ) d x + 1 2 I 3 Let u = x π 4 d u = d x = 0 π 4 ln ( 1 tan ( u + π 2 ) ) d u + 1 2 I 3 Note that tan ( π θ ) = tan θ = 0 π 4 ln ( 1 + tan ( π 2 u ) ) d u + 1 2 I 3 = 0 π 4 ln ( 1 + cot u ) d u + 1 2 I 3 = 0 π 4 ln ( tan u + 1 tan u ) d u + 1 2 I 3 = 0 π 4 ln ( 1 + tan u ) d u 0 π 4 ln ( tan u ) d u + 1 2 I 3 = I 3 I 4 \begin{aligned} I_2 & = \int_0^\frac \pi 2 \ln(1+\tan x)\ dx \\ & = {\color{#3D99F6}\int_\frac \pi 4^\frac \pi 2 \ln(1+\tan x)\ dx} + \int_0^\frac \pi 4 \ln (1+\tan x)\ dx & \small \color{#3D99F6} \text{Using identity: } \int_a^b f(x)\ dx = \int_a^b f(a+b-x)\ dx \\ & = {\color{#3D99F6}\int_\frac \pi 4^\frac \pi 2 \ln \left(1+\tan \left(\frac {3\pi}4 -x\right)\right)\ dx} + \frac 12 I_3 & \small \color{#3D99F6} \text{Note that }\tan (\pi - \theta) = - \tan \theta \\ & = {\color{#3D99F6}\int_\frac \pi 4^\frac \pi 2 \ln \left(1-\tan \left(x + \frac \pi 4\right)\right)\ dx} + \frac 12 I_3 & \small \color{#3D99F6} \text{Let }u = x - \frac \pi 4 \implies du = dx \\ & = \int_0^\frac \pi 4 \ln \left(1-\tan \left(u + \frac \pi 2\right)\right)\ du + \frac 12 I_3 & \small \color{#3D99F6} \text{Note that }\tan (\pi - \theta) = - \tan \theta \\ & = \int_0^\frac \pi 4 \ln \left(1+\tan \left(\frac \pi 2-u\right)\right)\ du + \frac 12 I_3 \\ & = \int_0^\frac \pi 4 \ln (1+\cot u)\ du + \frac 12 I_3 \\ & = \int_0^\frac \pi 4 \ln \left(\frac {\tan u + 1}{\tan u}\right)\ du + \frac 12 I_3 \\ & = \int_0^\frac \pi 4 \ln (1+\tan u)\ du - \int_0^\frac \pi 4 \ln (\tan u)\ du + \frac 12 I_3 \\ & = I_3 - I_4 \end{aligned}

I 4 = 0 π 4 ln ( tan x ) d x By integration by parts = x ln ( tan x ) 0 π 4 0 π 4 x sec 2 x tan x d x = 0 lim x 0 x ln ( tan x ) 0 π 4 x sin x cos x d x by L’H o ˆ pital’s rule = lim x 0 x 2 sec x tan x 1 2 0 π 4 x sin 2 x d x Let u = 2 x d u = 2 d x = lim x 0 x 2 sin x cos x 1 2 0 π 2 u sin u d u = lim x 0 x cos x 1 2 I 1 = 0 1 2 I 1 \begin{aligned} I_4 & = \int_0^\frac \pi 4 \ln (\tan x) dx & \small \color{#3D99F6} \text{By integration by parts} \\ & = x \ln (\tan x) \bigg|_0^\frac \pi 4 - \int_0^\frac \pi 4 \frac {x \sec^2 x}{\tan x} dx \\ & = 0 - {\color{#3D99F6}\lim_{x \to 0} x\ln(\tan x)} - \int_0^\frac \pi 4 \frac x{\sin x \cos x} dx & \small \color{#3D99F6} \text{by L'Hôpital's rule} \\ & = {\color{#3D99F6}\lim_{x \to 0} \frac {x^2\sec^ x}{\tan x}} - \color{#D61F06} \frac 12 \int_0^\frac \pi 4 \frac x{\sin 2x} dx & \small \color{#D61F06} \text{Let }u = 2x \implies du = 2\ dx \\ & = \lim_{x \to 0} \frac {x^2}{\sin x \cos x} - \color{#D61F06} \frac 12 \int_0^\frac \pi 2 \frac u{\sin u} du \\ & = \lim_{x \to 0} \frac {x}{\cos x} - \frac 12 I_1 \\ & = 0 - \frac 12 I_1 \end{aligned}

I 1 = 2 I 4 I_1 = - 2I_4

I 6 = 0 π 2 ln ( sin x ) d x By integration by parts = x ln ( sin x ) 0 π 2 0 π 2 x cos x sin x d x = 0 0 π 2 x tan x d x = I 5 \begin{aligned} I_6 & = \int_0^\frac \pi 2 \ln(\sin x)\ dx & \small \color{#3D99F6} \text{By integration by parts} \\ & = x \ln(\sin x) \bigg|_0^\frac \pi 2 - \int_0^\frac \pi 2 \frac {x\cos x}{\sin x} dx \\ & = 0 - \int_0^\frac \pi 2 \frac x{\tan x} dx \\ & = - I_5 \end{aligned}

I 7 = π 4 π 2 ln ( tan x ) d x = 0 π 2 ln ( tan x ) d x 0 π 4 ln ( tan x ) d x Using identity: a b f ( x ) d x = a b f ( a + b x ) d x = 1 2 0 π 2 [ ln ( tan x ) + ln ( tan ( π 2 x ) ) ] d x I 4 = 1 2 0 π 2 [ ln ( tan x ) + ln ( cot x ) ] d x I 4 = 0 I 4 \begin{aligned} I_7 & = \int_\frac \pi 4^\frac \pi 2 \ln(\tan x)\ dx \\ & = {\color{#3D99F6} \int_0^\frac \pi 2 \ln(\tan x)\ dx} - \int_0^\frac \pi 4 \ln(\tan x)\ dx & \small \color{#3D99F6} \text{Using identity: } \int_a^b f(x)\ dx = \int_a^b f(a+b-x)\ dx \\ & = {\color{#3D99F6} \frac 12 \int_0^\frac \pi 2 \left[\ln(\tan x) + \ln \left(\tan \left( \frac \pi 2 - x\right) \right)\right] dx} - I_4 \\ & = \frac 12 \int_0^\frac \pi 2 \left[\ln(\tan x) + \ln (\cot x)\right] dx - I_4 \\ & = 0 - I_4 \end{aligned}

Therefore, we have

I = I 1 + I 2 I 3 I 4 I 5 + I 6 + I 7 = 2 I 4 + I 3 I 4 I 3 I 4 I 5 I 5 I 4 = 4 I 4 I 4 = 4 \begin{aligned} I & = \frac {I_1+I_2-I_3-I_4}{I_5+I_6+I_7} = \frac {-2I_4+I_3-I_4-I_3-I_4}{I_5-I_5-I_4} = \frac {-4I_4}{-I_4} = \boxed{4} \end{aligned}

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