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3.Trigonometrical Ratios, Functions and Identities
easy
જો $A + B + C = \pi $ અને $\cos A = \cos B\,\cos C,$ તો $\tan B\,\,\tan C $ = . . .
A
$\frac{1}{2}$
B
$2$
C
$1$
D
$ - \frac{1}{2}$
Solution
(b) $\cos [\pi – (B + C)] = \cos B\cos C$
$⇒$ $ – \cos (B + C) = \cos B\cos C$
$⇒$ $ – [\cos B\cos C – \sin B\sin C] = \cos B\cos C$
$⇒$ $\sin B\sin C = 2\cos B\cos C$
$⇒$ $\tan B\tan C = 2$.
Standard 11
Mathematics
