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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Trigonometric Equations

NDA 2025 · Mathematics · Trigonometric Equations

If 0 ≤ x ≤ (π)/(2) , then what is the number of values of x...

If 0≤x≤π20 \leq x \leq \dfrac{\pi}{2}, then what is the number of values of xx satisfying the equation tan⁡x+sec⁡x=2cos⁡x\tan x + \sec x = 2\cos x?

Options

  1. A00
  2. B11
  3. C22
  4. D33

Correct answer

B. 11

Step-by-step solution

The given equation is tan⁡x+sec⁡x=2cos⁡x\tan x + \sec x = 2\cos x.

Rewriting the equation in terms of sin⁡xandcos⁡x\sin x and \cos x:

sin⁡xcos⁡x+1cos⁡x=2cos⁡x\dfrac{\sin x}{\cos x} + \dfrac{1}{\cos x} = 2\cos x

For the equation to be defined, cos⁡x≠0\cos x \neq 0, which means x≠π2x \neq \dfrac{\pi}{2}. Thus, x \in \left[0, \dfrac{\pi}{2}\right).<br><br>Multiplying both sides by\cos x:<br><br>:<br><br>\sin x + 1 = 2\cos^2 x<br><br>Substituting<br><br>Substituting\cos^2 x = 1 - \sin^2 x:<br><br>:<br><br>\sin x + 1 = 2(1 - \sin^2 x)<br><br><br><br>2\sin^2 x + \sin x - 1 = 0<br><br>Factoringthequadraticequation:<br><br><br><br>Factoring the quadratic equation:<br><br>(2\sin x - 1)(\sin x + 1) = 0<br><br>Thisgives<br><br>This gives\sin x = 12orsin⁡x=−1\dfrac{1}{2} or \sin x = -1.

Since x∈[0,π2)x \in \left[0, \dfrac{\pi}{2}\right), sin⁡x≥0\sin x \geq 0. Therefore, sin⁡x=−1\sin x = -1 is rejected.

For sin⁡x=12\sin x = \dfrac{1}{2}, the only solution in the given interval is x=π6x = \dfrac{\pi}{6}.

Thus, there is exactly 11 value of xx satisfying the equation.

Answer: 11
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