Because alpha +beta=180 then cos(alpha)=-cos(beta), so you can make an equation using results from the first part.
First part of the question was easy,
The second part is where i'm stuck.
What I have tried :
-Make d the subject in both cases and have one equal to each other (All that happens is that everything cancels each other out and I get 0=0)
-Make d^2 the subject (I don't know why I tried this, same result as before >.>)
Completely lost as to how to solve this problem.
Any help would be appreciated!
(Sorry for poor diagram etc. in advance)
Click the image to make it larger.