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Homework Help: Confused about proof of "sin(θ + Φ) = cosθsinΦ + sinθcosΦ"

  1. Oct 27, 2015 #1
    This is also a sort of geometry question.
    My textbook gives a proof of the relation: sin(θ + Φ) = cosθsinΦ + sinθcosΦ.
    It uses a diagram to do so:

    sin (θ + Φ) = PQ/(OP)
    = (PT + RS)/(OP)
    = PT/(OP) + RS/(OP)
    = PT/(PR) * PR/(OP) + RS/(OR) * OR/(OP)
    = cosθsinΦ + sinθcosΦ

    My confusion with this is
    How do they know that angle TPR also measures θ?
    How do they know that triangle TPR is similar to triangle ROQ?


    The textbook is: Calculus with Analytic Geometry, 2e by George F. Simmons
  2. jcsd
  3. Oct 27, 2015 #2

    I think I kind of figured it out, angle ros = theta, angle ors = 90 -θ

    the angle between the TR and the brown line is theta, the angle between PR and the brown line is 90degrees right so angle PRT is 90 - theta and so TPR is theta i've attached a diagram because I feel like these words aren't making sense. Is the diagram clear?

    They are similar triangles because they have the same angles.

    Attached Files:

  4. Oct 27, 2015 #3


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    They use geometry.

    I assume they intend for ∠ORP to be a right angle.

    ∠RTO measures θ. ∠TRP measures 90° - θ . etc.

    (I assume you meant ΔROS, not ΔROQ .)
    As for ΔTPR and ΔROS, they're both right triangles each having an acute angle with the same measure, namely θ.
  5. Oct 27, 2015 #4
    Oh, wow thanks. It makes sense now. Thanks especially for taking the time to upload a picture.
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