If 3 = + ^3 , then ( , ) = - Vidyadip

MCQ

If $\cos 3\theta = \alpha \cos \theta + \beta {\cos ^3}\theta ,$ then $(\alpha ,\beta ) = $

A
$(3,\,4)$
B
$(4,\,3)$
✓
$( - 3,\,4)$
D
$(3,\, - 4)$

✓

Answer

Correct option: C.

$( - 3,\,4)$

c
(c) Given that $\cos 3\theta $

$= \alpha \cos \theta + \beta {\cos ^3}\theta $

But $\cos 3\theta = 4{\cos ^3}\theta - 3\cos \theta $

==> $(\alpha ,\beta ) = ( - 3,\,4)$.

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