Cosine rule problem

  • Thread starter CAH
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  • #1
CAH
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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).

Thanks
 

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  • #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##
 
  • #3
CAH
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Still don't see what to do next
 
  • #4
SammyS
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Still don't see what to do next
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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