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$\cos 6 x = 32 ° \cos x - k \cos^{4} x + 18 ° \cos^{2} x - 1$ then find k =

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answer is B.

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Detailed Solution

$\begin{array}{l} \cos 6 x \\ = \cos 3 (2 x) \\ = 4 \cos^{3} 2 x - 3 \cos 2 x \\ = 4 {(2 \cos^{2} x - 1)}^{3} - 3 (2 \cos^{2} x - 1) \\ = 32 \cos^{6} x - 48 \cos^{4} x + 18 \cos^{2} x - 1 \end{array}$

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