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Homework Help: Equation problem.

  1. Jan 20, 2014 #1
    1. The problem statement, all variables and given/known data

    Determine the power generated by a voltage V = 240∠15° volts driving a current I = 12 ∠75°

    2. Relevant equations


    3. The attempt at a solution This is a worked example from my open university course. The answer is 240x12cos(15°-75°) = 1440W. I can not for the life in me work out how they have arrived at this answer. I get cos-60= 0.58. then times 240x12= 1692.9 W. Must be doing something wrong!
  2. jcsd
  3. Jan 20, 2014 #2


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    Staff: Mentor

    How is cos(60°) equal to 0.58?
  4. Jan 20, 2014 #3


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    Homework Helper

    It is possible that the OP has their calculator in GRAD mode, since cos(60 grad) = 0.588.

    But really one shouldn't need a calculator to calculate cos(60°) = 0.5.
  5. Jan 20, 2014 #4
    Not sure. After inputting cos 60 it came out with 0.58. I am of the understanding cosine 60 equals half. so 240x12x0.5 does equal 1440 Watts . But cosine 60 equates to 0.58 in my calculator.
  6. Jan 20, 2014 #5


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    Staff: Mentor

    As pasmith indicated, check to make sure that your calculator is in degree mode for angles, not grad mode.
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