# Cosine rule problem

1. Apr 26, 2015

### CAH

• 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. Apr 26, 2015

### Simon Bridge

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. Apr 27, 2015

### CAH

Still don't see what to do next

4. Apr 27, 2015

### SammyS

Staff Emeritus
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: