Cosine Rule Problem: I Can't Do a(ii) - Get Help Here

[CAH]

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

 

[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: $\cos(A+B)\neq \cos A + \cos B$

 

[CAH]

Still don't see what to do next

 

[SammyS]

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.