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Questions  >>  CBSE XI  >>  Math  >>  Trigonometric Functions
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Q)

$\cos 2\theta\cos 2\phi+\sin^2(\theta-\phi)-\sin^2(\theta+\phi)$ is equal to

$\begin{array}{1 1}(A)\;\sin 2(\theta+\phi)&(B)\;\cos 2(\theta+\phi)\\(C)\;\sin 2(\theta-\phi)&(D)\;\cos 2(\theta-\phi)\end{array} $

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A)
Toolbox:
  • $\sin^2A-\sin^2B=\sin(A+B).\sin (A-B)$
$\cos 2\theta\cos 2\phi+\sin^2(\theta-\phi)-\sin^2(\theta+\phi)$
$\cos 2\theta\cos 2\phi+\sin 2\theta.\sin(-2\phi)$
$\cos 2\theta\cos 2\phi-\sin 2\theta.\sin2\phi$
$\cos (2\theta+2\phi)$
$\cos 2(\theta+\phi)$
Hence (B) is the correct answer.
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