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Math Help - Need help with yet another trigo identity

  1. #1
    Junior Member
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    Aug 2010
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    Need help with yet another trigo identity

    Hi Guys !

    Here is yet another trigo identity which i simply can't figure it out

    please help thanks !

    sinA + sinB + sin(A+B) = 4sin{ (A+B)/2 }cos(A/2)cos(B/2)
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  2. #2
    MHF Contributor
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    1. Rewrite \sin\alpha+\sin\beta using the sum-to-product rule.

    2. Rewrite \sin(\alpha+\beta) as \sin(2(\alpha+\beta)/2) and further using the formula for double angle.

    3. Factor out 2\sin((\alpha+\beta)/2) in the left-hand side.

    4. To the remaining sum of cosines in the left-hand side, apply the sum-to-product rule.
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