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Math Help - Help - Sum of Cosines

  1. #1
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    Help - Sum of Cosines

    I need help with the sum of cosines. Specifically this problem:

    Cos(2pi/3 + pi/4)

    Detail is highly appreciated. Thank you in advance.
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  2. #2
    Senior Member apcalculus's Avatar
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    Quote Originally Posted by Neversh View Post
    I need help with the sum of cosines. Specifically this problem:

    Cos(2pi/3 + pi/4)

    Detail is highly appreciated. Thank you in advance.
    cos (a + b ) = cos(a)cos(b) - sin(a)sin(b)

    a= 2pi/3

    b = pi/4

    ...and you definitely know the sines and cosines of these two angles.

    Good luck!
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  3. #3
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    Had to finish dinner. So, the awnser would be: (-sqrt(2) - sqrt(6))/4 ?
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  4. #4
    Senior Member apcalculus's Avatar
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    Quote Originally Posted by Neversh View Post
    Had to finish dinner. So, the awnser would be: (-sqrt(2) - sqrt(6))/4 ?
    correct.
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