I am having some problems with these questions,can anybody help?

2) It is given that sinA = 5/13 where 90<A<180(degrees) , and that cosB = -33/65 , where 180<B<270. Find the exact value of cos2(A+B)

How do i expand cos2(A+B) ? apparently when i use my calculator cos2(A+B) = cos(2A + 2B) but again,the expansion gets too complicated and i derived a six digit fraction which is a far cry from the answer (7/25)

3)A right pyramid has a square horizontal base ABCD of side 2x and a vertex O. Given that Angle AOB = $\displaystyle 2\theta$ , show that the inclination of OA to the horizontal is $\displaystyle arcsin(\sqrt cos2\theta)$ and find the inclination of the plane OAB to the horizontal.

I can't think of any way to start on this problem where I am able to ultilize all of the information given.

1)(solved) If $\displaystyle A + B + C = \dfrac{\pi}{2} \,radians $ , show that $\displaystyle tanA tanB + tanB tanC + tanC tanA = 1 $

For this I was thinking to make the pi/2 into 1 we have to introduce sine to A+B+C but it just gets more and more complicated when i try to expand everything and I feel that there should be a simpler way which I have overlooked.

Any help at all is appreciated