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Logarithm question

2020 · 9 Jan · Shift 1 · Q34
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  5. /2020 · 9 Jan · Shift 1 · Q34

Logarithm question

2020 · 9 Jan · Shift 1 · Q34

JEE MainMathematicsLogarithmNumerical+4 / −1
The number of distinct solutions of the equation log⁡12∣sin⁡x∣=2−log⁡12∣cos⁡x∣{\log _{{1 \over 2}}}\left| {\sin x} \right| = 2 - {\log _{{1 \over 2}}}\left| {\cos x} \right|log21​​∣sinx∣=2−log21​​∣cosx∣ in the interval [0, 2 π\piπ], is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 8

  1. Given equation

    log⁡12∣sin⁡x∣=2−log⁡12∣cos⁡x∣\log_{\frac12}|\sin x| = 2 - \log_{\frac12}|\cos x|log21​​∣sinx∣=2−log21​​∣cosx∣

    We need the number of distinct solutions in [0,2π][0,2\pi][0,2π].

  2. Bring logarithms together

    Move the logarithmic term on the right to the left:

    log⁡12∣sin⁡x∣+log⁡12∣cos⁡x∣=2\log_{\frac12}|\sin x| + \log_{\frac12}|\cos x| = 2log21​​∣sinx∣+log21​​∣cosx∣=2

    Using log⁡am+log⁡an=log⁡a(mn),\log_a m + \log_a n = \log_a(mn),loga​m+loga​n=loga​(mn), we get

    log⁡12(∣sin⁡x∣ ∣cos⁡x∣)=2\log_{\frac12}(|\sin x|\,|\cos x|)=2log21​​(∣sinx∣∣cosx∣)=2

    Hence,

    ∣sin⁡x∣ ∣cos⁡x∣=(12)2=14|\sin x|\,|\cos x| = \left(\frac12\right)^2 = \frac14∣sinx∣∣cosx∣=(21​)2=41​

  3. Simplify using double-angle identity

    Since 2∣sin⁡x∣∣cos⁡x∣=∣sin⁡2x∣,2|\sin x||\cos x| = |\sin 2x|,2∣sinx∣∣cosx∣=∣sin2x∣, we get

    ∣sin⁡2x∣=2⋅14=12|\sin 2x| = 2\cdot \frac14 = \frac12∣sin2x∣=2⋅41​=21​

    So we must solve

    ∣sin⁡2x∣=12|\sin 2x| = \frac12∣sin2x∣=21​

  4. Solve the trigonometric equation

    The equation ∣sin⁡θ∣=12|\sin\theta|=\frac12∣sinθ∣=21​ has solutions in one full cycle [0,2π][0,2\pi][0,2π]:

    θ=π6, 5π6, 7π6, 11π6\theta = \frac\pi6,\ \frac{5\pi}6,\ \frac{7\pi}6,\ \frac{11\pi}6θ=6π​, 65π​, 67π​, 611π​

    Here θ=2x\theta=2xθ=2x. Since x∈[0,2π]x\in[0,2\pi]x∈[0,2π], we have 2x∈[0,4π]2x\in[0,4\pi]2x∈[0,4π], i.e. two full cycles.

    Therefore the number of solutions for 2x2x2x is 4×2=84\times 2 = 84×2=8.

    Corresponding values of xxx are:

    x=π12, 5π12, 7π12, 11π12, 13π12, 17π12, 19π12, 23π12x=\frac\pi{12},\ \frac{5\pi}{12},\ \frac{7\pi}{12},\ \frac{11\pi}{12},\ \frac{13\pi}{12},\ \frac{17\pi}{12},\ \frac{19\pi}{12},\ \frac{23\pi}{12}x=12π​, 125π​, 127π​, 1211π​, 1213π​, 1217π​, 1219π​, 1223π​

  5. Check domain restrictions

    Since logarithms require ∣sin⁡x∣>0and∣cos⁡x∣>0,|\sin x|>0 \quad \text{and} \quad |\cos x|>0,∣sinx∣>0and∣cosx∣>0, we must exclude points where sin⁡x=0\sin x=0sinx=0 or cos⁡x=0\cos x=0cosx=0.

    None of the 8 values above make sin⁡x\sin xsinx or cos⁡x\cos xcosx zero, so all are valid.

  6. Conclusion

    The number of distinct solutions is

    8\boxed{8}8​

Previous

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