Question

x ∈ ($\pi$/2 ; 3$\pi$/4) astfel incat cos 2x=-$\sqrt{3}$/2; cos x=?

Answer 1

cos 2x=-(V3)/2
 cos2x=2(cosx)^2-1

2(cosx)^2-1=-(V3)/2
2(cosx)^2=1-(V3)/2
2(cosx)^2=(2-V3)/2
(cosx)^2=(2-V3)/4
cosx=V[(2-V3)/4]
cosx=V(2-V3)/2