# Cosine rule problem

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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Simon Bridge
Homework Helper
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##

Still don't see what to do next

SammyS
Staff Emeritus
Homework Helper
Gold Member
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: