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Math Help - Compound Angle Formula.

  1. #1
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    Compound Angle Formula.

    I just started learning Trig identities and i've never been so confused in my life! Can someone please explain how to do this?


    Prove: cos (A-B) - cos (A+B) = 2sin (A) sin (B)

    Thanks.
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  2. #2
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    Quote Originally Posted by Tina7906 View Post
    I just started learning Trig identities and i've never been so confused in my life! Can someone please explain how to do this?


    Prove: cos (A-B) - cos (A+B) = 2sin (A) sin (B)

    Thanks.
    Hi, Tina, welcome to MHF,
    According to the compound angle formula expand
    cos(A-B) = cosA*cosB +sinA*sinB.
    Similarly expand cos(A+B) and add,
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  3. #3
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    Thank you!

    I did that at first and that's where i got stuck.
    Im not actually sure how to add and make it equal 2sin (A) sin (B).

    Would you be able to explain further?
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  4. #4
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    Quote Originally Posted by Tina7906 View Post
    Thank you!

    I did that at first and that's where i got stuck.
    Im not actually sure how to add and make it equal 2sin (A) sin (B).

    Would you be able to explain further?
    cos(A+B) = cosA*cosB -sinA*sinB
    cos(A-B) = cosA*cosB + sinA*sinB
    So
    cos(A-B) - cos(A+B) = 2sinA*sinB
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