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2015-02-17T10:01:41+08:00
Double‐Angle and Half‐Angle IdentitiesSpecial cases of the sum and difference formulas for sine and cosine yields what is known as the double‐angle identities and the half‐angle identities. First, using the sum identity for the sine,                                                                       sin 2α = sin (α + α)sin 2α = sin α cos α + cos α sin αsin 2α = 2 sin α cos α

Similarly for the cosine,
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