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Trigonometrical Equations
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The number of solutions that the equation $sin5\theta cos3\theta = sin9\theta cos7\theta $ has in $\left[ {0,\frac{\pi }{4}} \right]$ is
A
$4$
B
$5$
C
$6$
D
$7$
Solution
$\sin 5 \theta \cos 3 \theta=\sin 9 \theta \cos 7 \theta$
$\Rightarrow 2 \sin 5 \theta \cos 3 \theta=2 \sin 9 \theta \cos 7 \theta$
$\Rightarrow \sin 8 \theta+\sin 2 \theta=\sin (16 \theta)+\sin 2 \theta$
$\Rightarrow \sin 8 \theta-\sin 16 \theta=0$
$\Rightarrow \sin 8 \theta(1-2 \cos 8 \theta)=0$
$\Rightarrow \sin 8 \theta=0$ or $\cos 8 \theta=\frac{1}{2}$
$\Rightarrow 8 \theta=n \pi$ or $8 \theta=2 n \pi \pm \frac{\pi}{3}$
$\Rightarrow \theta=\frac{n \pi}{8}$ or $\theta=\frac{2 n \pi}{8} \pm \frac{\pi}{24}$
Therefore number of values of $\theta$ is 5
Standard 11
Mathematics