2-2cos²x+cosx-1=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,n∈z
-5π≤-2π/3+2πn≤-4π
-15≤-2+6n≤-12
-13≤6n≤-10
-13/6≤n≤-10/6
n=-2⇒x=-2π/3-4π=-14π/3
-5π≤2π/3+2πn≤-4π
-15≤2+6n≤-12
-17≤6n≤-14
-17/6≤n≤-14/6
нет решения
a2=(1+3)/4=1⇒cosx=1⇒x=2πk,k∈z
-5π≤2πk≤-4π
-5/2≤k≤-2
k=-2⇒x=-4π