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}