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How to use double angle formula to write $8 {cos}^{2} x - 4$ $8cos^2x-4$ ?

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Apr 29, 2015

Use trig identity: $cos 2 a = 2 {cos}^{2} a - 1$ $cos 2a = 2cos^2 a - 1$

$y = 2 . [2 . {cos}^{2} (x) - 1] = 2 . cos 2 x$ $y = 2.[2.cos^2 (x) - 1] = 2.cos 2x$

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