Prove that- cos -7 cos 2-7 cos 4-7 - -18

L.H.S = 12^3 sin (π/7)· sin (π + π7 ) = 18 sin (π/7)· sin ( π + π7 ) = 18 ∵ sin(π + θ) = sin θ ...

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De-Moivre's Theorem

Prove that: ...

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Prove that: $cos \frac{π}{7} cos \frac{2 π}{7} cos \frac{4 π}{7} = - \frac{1}{8}$

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cos \frac{π}{7} cos \frac{2 π}{7} cos \frac{4 π}{7} = ?

cos \frac{π}{7} cos \frac{2 π}{7} cos \frac{4 π}{7} =

Write the value of $c o s \frac{π}{7} c o s \frac{2 π}{7} c o s \frac{4 π}{7}$ .

cos \frac{π}{7} cos \frac{2 π}{7} cos \frac{4 π}{7} = \frac{1}{8}

cos θ cos 2 θ cos 4 θ . . . cos 2^{n - 1} θ = \frac{1}{2^{n}}

θ = \frac{π}{2^{n} - 1}

cos \frac{π}{7} cos \frac{2 π}{7} cos \frac{4 π}{7} = - \frac{1}{8}

cos θ cos 2 θ cos 4 θ . . . cos 2^{n - 1} θ = - \frac{1}{2^{n}}

θ = \frac{π}{2^{n} - 1}

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