Помогите с синусами и косинусами (

а) 2*cos²x+cos2x=2cosx+1

б) Найти все корни на промежутке [-2π; -π/2]

2cos²x+2cos²x-1-2cosx-1=0

4cos²x-2cosx-2=0

2cos²x-cosx-1=0

cosx=a

2a²-a-1=0

D=1+8=9

a1 = (1-3) / 4=-1/2⇒cosx=-1/2⇒x=+-2π/3+2πn

-2π≤-2π/3+2πn≤-π/2 U - 2π≤2π/3+2πn≤-π/2

-4π/3≤2πn≤π/6 U - 8π/3≤2πn≤-7π/6

-2/3≤n≤1/12 U - 4/3≤n≤-7/12

n=0⇒x=-2π/3 U n=-1⇒x=-π/3

a2 = (1+3) / 4=1⇒cosx=1⇒x=2πn

-2π≤2πn≤-π/2

-1≤n≤-1/4

n=-1⇒x=-2π

Ответx={-2π/3; -π/3; -2π}