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    GorillaLovers22's Avatar
    GorillaLovers22 Posts: 1, Reputation: 1
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    #1

    Apr 20, 2012, 06:06 PM
    Trig Identity
    I need helping figureing out cos3x/cosx = 1-4sin^2x.
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Apr 23, 2012, 08:20 AM
    Replace cos(3x) with cos(x+2x), using the identity cos(a+b) = cos(a)cos(b)-sin(a)sin(b). The result will have include sin(2x) and cos(2x) - replace these with their identities in terms of cos(x) and sin(x).
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