Trig pls help

Please re-format your question so it is correct....  is that SUPPOSED to be   - 1/5

                                                                               is that SUPPOSED to be cos (2x)   or  cos (x/2) or cos^2 x ?

I think this is the trig identity you may need:

If angle x is in Q III...then x/2 will be in Q I    and will be + POSITIVE    in value