Trigonometrical Equations
normal

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

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