Hi

2) Knowing you can compute

Then x being in quadrant III, you can compute

Then

4) 2 possibilities (at least)

a) Chose some values (x=0, x=pi/2, ...) to get 3 linear equations with unknowns A, B and C

b)

Therefore you can find the expression of and find A, B and C such that

5)

and A-\sqrt{B})" alt="\cos \frac{\pi}{6} = 1 - 2 \sin^2 \frac{\pi}{12} = 1 - \frac12\A-\sqrt{B})" />