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We want to prove `sin8A=8sinA*cosA*cos2A*cos4A`
`=sin4Acos4A+sin4A+cos4A` :sum of angles identity
`=2(2sin2Acos2Acos4A)` ``: double angle identity
`=4(2sinAcosAcos2Acos4A)` : double angle identity
`=8sinAcosAcos2Acos4A` as required.
** In general, when proving trigonometric identities you start with one side, and by using allowable algebraic manipulations and other trigonometric identities, you transform that side into the other side.
Posted by embizze on June 22, 2012 at 3:45 PM (Answer #1)
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