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Homework Help: Cosine rule problem

  1. Apr 26, 2015 #1


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    • Member warned about posting with no template, and posting only images
    See the photo attachments of question and marking scheme and my attempt at a solution :)

    I've done a(i) but I can't do a(ii).


    Attached Files:

  2. jcsd
  3. Apr 26, 2015 #2

    Simon Bridge

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    For a(ii) you have, basically ##\theta+\phi=\pi## (always use radians) ... with theta and phi expressed as inverse cosines of functions of x.
    Your next step was to take the cosine of both sides - but you did this step incorrectly:
    note: ##\cos(A+B)\neq \cos A + \cos B##
  4. Apr 27, 2015 #3


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    Still don't see what to do next
  5. Apr 27, 2015 #4


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    You could go back and fix things up as Simon suggested.

    I would suggest that there was little point in taking the arccos when you did.

    Notice that:

    π - ∠ADB = ∠ADC

    Take the cosine of both sides of this. As Simon says, use a correct form of angle difference formula for cosine.
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