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Math Help - Identity Crisis!

  1. #1
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    Identity Crisis!

    Hi, Could someone please help me prove this identity?

    sinA + sin9A = 2.sin5A.cos4A

    Thanks, your help would be very much appreciated!
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  2. #2
    MHF Contributor
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    Quote Originally Posted by sammysparrow View Post
    Hi, Could someone please help me prove this identity?

    sinA + sin9A = 2.sin5A.cos4A

    Thanks, your help would be very much appreciated!
    Use the trigonometric identity

    \displaystyle\ SinA+SinB=2Sin\left(\frac{A+B}{2}\right)Cos\left(\  frac{A-B}{2}\right)

    with B=9A

    coupled with Cos(-C)=CosC
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  3. #3
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    Thanks Archie Meade!
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