Q12 / 83 · NTA Abhyas (JEE Main) 2020 · Mathematics · Trigonometric Equations

Let 0<θ1<θ2<θ3<… denotes the positive solutions of the equation 3+3cos⁡θ=2sin2⁡θ. If θ3+θ7=aπ, where a is an integer, then the value of a is equal to

  • Correct answer
    A
    6
  • B
    7
  • C
    8
  • D
    4

Solution

3+3cos⁡θ=2sin2⁡θ=2-2cos2⁡θ.

⇒2cos2⁡θ+3cos⁡θ+1=0

⇒cos⁡θ=-1,-12

⇒θ=2n+1π or 2nπ±2π3

⇒θ=2π3,π,4π3,8π3,3π,10π3,14π3,5π

i.e. θ3+θ7=4π3+14π3=18π3=6π

⇒a=6

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

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