Class 11MathematicsTrigonometrical Ratios, Functions and IdentitiesMix Examples-Trigonometrical Ratios, Functions and Identities
MediumProve that: $(\sin 3x + \sin x) \sin x + (\cos 3x - \cos x) \cos x = 0$
(N/A) $L.H.S.$ $= (\sin 3x + \sin x) \sin x + (\cos 3x - \cos x) \cos x$
$= \sin 3x \sin x + \sin^2 x + \cos 3x \cos x - \cos^2 x$
$= (\cos 3x \cos x + \sin 3x \sin x) - (\cos^2 x - \sin^2 x)$
$= \cos(3x - x) - \cos 2x$
$= \cos 2x - \cos 2x$
$= 0$
$= R.H.S.$
$= \sin 3x \sin x + \sin^2 x + \cos 3x \cos x - \cos^2 x$
$= (\cos 3x \cos x + \sin 3x \sin x) - (\cos^2 x - \sin^2 x)$
$= \cos(3x - x) - \cos 2x$
$= \cos 2x - \cos 2x$
$= 0$
$= R.H.S.$
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