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BITSAT 2014
Exam
BITSAT
Year
2014
Subject
Mathematics
Chapter
Inverse Trigonometric Functions

BITSAT 2014 · Mathematics · Inverse Trigonometric Functions

Find the value of [2 ^ -1 (1)/(5)-(π)/(4)]

Find the value of \tan \left[2 \tan ^{-1}$\frac{1}{5}-\frac{\pi}{4}\right]

Options

  1. A−1/3-1 / 3
  2. B−7/17-7 / 17
  3. C−1/2-1 / 2
  4. D−1/4-1 / 4

Correct answer

B. −7/17-7 / 17

Step-by-step solution

2tan⁡−115=tan⁡−115+tan⁡−115=tan⁡−115+151−15⋅15=tan⁡−12/524/25=tan⁡−1512=tan⁡(2tan⁡−115−π4)=tan⁡(tan⁡−1512−π4)=tan⁡(tan⁡−1512)−tan⁡π41+tan⁡(tan⁡−1512)tan⁡π4=−7121712=−717\begin{aligned} & 2 \tan ^{-1} \frac{1}{5}=\tan ^{-1} \frac{1}{5}+\tan ^{-1} \frac{1}{5} \\ &=\tan ^{-1} \frac{\frac{1}{5}+\frac{1}{5}}{1-\frac{1}{5} \cdot \frac{1}{5}}=\tan ^{-1} \frac{2 / 5}{24 / 25}=\tan ^{-1} \frac{5}{12} \\ &=\tan \left(2 \tan ^{-1} \frac{1}{5}-\frac{\pi}{4}\right)=\tan \left(\tan ^{-1} \frac{5}{12}-\frac{\pi}{4}\right) \\ &=\frac{\tan \left(\tan ^{-1} \frac{5}{12}\right)-\tan \frac{\pi}{4}}{1+\tan \left(\tan ^{-1} \frac{5}{12}\right) \tan \frac{\pi}{4}}=\frac{-\frac{7}{12}}{\frac{17}{12}}=-\frac{7}{17} \end{aligned}
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