Q9 / 83 · NTA Abhyas (JEE Main) 2020 · Mathematics · Trigonometric Equations

The number of solutions of the equation log2sin⁡x(1+cos⁡x)=2 in the interval 0,5π is

Correct answer: 3

Solution

log2sin⁡x1+cos⁡x=2
1+cos⁡x=2sin⁡x2=2sin2⁡x
1+cos⁡x=2-2cos2⁡x
2cos2⁡x+cos⁡x-1=0
⇒cos⁡x+12cos⁡x-1=0
⇒cos⁡x=12 or cosx=-1 (rejected)
⇒x=π3,5π3 in 0,2π
But at x=5π3, 2sin⁡x is negative
⇒x=π3 in 0,2π
⇒x=π3, π3+2π, π3+4π
⇒3 solutions in 0,5π

Hence, the correct answer is 3.

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