Answer:
180< x < 270 so x is in 3rd quadrant ,hence tan x is positive cos x=-4/5 tan(x/2) = √ {(1 − cos x) / (1 + cos x)) = √ { (1+4/5)/(1–4/5) } =√{ 9/5)/(1/5) } =√9=±3 now ...
