3.Trigonometrical Ratios, Functions and Identities
easy

यदि $A + B + C = \pi $ तथा $\cos A = \cos B\,\cos C,$ तब $\tan B\,\,\tan C$ का मान होगा

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

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