Q40 / 87 · NTA Abhyas (JEE Main) 2020 · Mathematics · Functions

For x∈-π2,π2, the range of values of fx=2+sin⁡x+sin3⁡x+sin5⁡x…..∞ is

  • A
    0,1
  • Correct answer
    B
    -∞,∞
  • C
    -2,2
  • D
    None of these

Solution

We have, fx=2+sin⁡xcos2⁡x

⇒ f ′ x = cos 2 x cosx +sinx 2cosxsinx cos 4 x

=cos⁡xcos2⁡x+2sin2⁡xcos4⁡x=1+sin2⁡xcos3⁡x

⇒ f ′ x >0

∴fx is an increasing function

limx→-π2⁡2+sin⁡xcos2⁡x=-∞

and limx→π2⁡2+sin⁡xcos2⁡x=∞

∴ Range =-∞, ∞

Hence, the correct answer is option: 'B'.

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