Question

2cos^x+5 cosx+2=0
1-cos2x= ( sin 2 x- cos2 x)^

Answer 1

2cos²x+5cosx+2=0   Пусть cosx=v    ⇒  2v²+5v+2=0  D=3  v₁=-2  v₂=-0,5
⇒  cosx=-2     x∉
     cosx=-1/2  x=-π/3+2πn