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Trig average power Question

  1. Apr 26, 2008 #1
    1. The problem statement, all variables and given/known data

    8sin(200t) - 6 cos(200t - pi/4)

    2. Relevant equations

    3. The attempt at a solution
    I'm trying to calculate the average power for that expression so:
    I need to get that expression into either one term with the same frequency and phase angle or two terms with a different frequency.
    I've tried a bunch of different methods but can't quite get anything..

    I've tried to turn 8sin(200t) into 4cos(100t)sin(100t) = 4cos(100t)cos(100t - pi/2) but then I get stuck with cosines multiplying each other.

    Any help?
  2. jcsd
  3. Apr 26, 2008 #2

    Tom Mattson

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    So is this supposed to be the instantaneous power?
  4. Apr 26, 2008 #3
    Thanks for your reply,

    Sorry I kind of misspoke, it's actually instantaneous voltage,
    vs = 8sin(200t) - 6cos(200t - 45 deg)
    R = 4 ohms

    P = 1/2 VI cos(theta - phi)

    I think the best way to approach it would be to find it as an expression with two terms of different frequencies (so I can use superposition) but so far that hasn't worked
  5. Apr 26, 2008 #4

    Tom Mattson

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    You should decompose the second term using the following identity.


    Then all of your time-dependent trig functions will have the same phase and frequency.
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