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3.Trigonometrical Ratios, Functions and Identities
easy
$\sin \left( {\frac{\pi }{{10}}} \right)\sin \left( {\frac{{3\pi }}{{10}}} \right) = $
A
$1/2$
B
$-1/2$
C
$1/4$
D
$1$
Solution
(c) $\sin \frac{\pi }{{10}}\sin \frac{{3\pi }}{{10}} = \sin 18^\circ .\sin 54^\circ $
$ = \sin 18^\circ .\cos 36^\circ = \frac{{\sqrt 5 – 1}}{4}.\frac{{\sqrt 5 + 1}}{4} = \frac{1}{4}$.
Standard 11
Mathematics
