यदि cos theta = frac{1}{2}left( {a + frac{1}{ | vedclass

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(c) $\because$ $\;\cos 3	heta  = 4{\cos ^3}	heta  - 3\cos 	heta $ 

$	herefore \cos 3	heta = 4\frac{1}{{{2^3}}}{\left(

Vedclass · Accepted Answer

$\frac{1}{2}\left( {{a^3} + \frac{1}{{{a^3}}}} \right)$

Vedclass · Answer

(c) $\because$ $\;\cos 3	heta  = 4{\cos ^3}	heta  - 3\cos 	heta $ 

$	herefore \cos 3	heta = 4\frac{1}{{{2^3}}}{\left( {a + \frac{1}{a}} ight)^3} - 3\frac{1}{2}\left( {a + \frac{1}{a}} ight)$

$ \Rightarrow \cos 3\,	heta = \frac{1}{2}\left( {a + \frac{1}{a}} ight)\,\left[ {{{\left( {a + \frac{1}{a}} ight)}^2} - 3} ight]$ 

==> $\cos 3	heta = \frac{1}{2}\left( {{a^3} + \frac{1}{{{a^3}}}} ight)$.

यदि $\cos \theta = \frac{1}{2}\left( {a + \frac{1}{a}} \right),$ तब $\cos 3\theta $ का मान होगा

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