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INTEGRALS — Maths STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceMathsINTEGRALS4 Marks
Question
Using properties of definite integrals, evaluate:

$\int\limits^{\pi/4}_{0}\text{log (1 + tan x) dx}$.

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Answer

$\text{I}=\int_{0}^{\pi/4}\log\text{(1 + tan x ) dx}=\int_0^{\pi/4}\log\Bigg(1+\tan(\frac{\pi}{4}-\text{x})\Bigg)\text{dx}$

​​​​​​$=\int_{0}^{\pi/4}\log\Bigg(1+\frac{1-\tan\text{x}}{\text{1 + tan x}}\Bigg)\text{dx}=\int_{0}^{\pi/4}\log\Bigg(\frac{2}{\text{1 + tan x}}\Bigg)\text{dx}$

$=\int_{0}^{\pi/4}\log2\text{ dx}-\int_0^{\pi/4}\log(1+\tan\text{ x})\text{ dx}$

$\Rightarrow\text{2I}=\log2\cdot\int_0^{\pi/4}1\cdot\text{dx}=\log2\cdot\pi/4$

$\Rightarrow\text{I}=\pi/8\cdot\log2.$

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