)20 cos2x= -1 sin 2x=1/2 sinx-√2/2=0 cosx-√2/2=0

1) 2x = πk, k∈ Z,
    x = πk/2 , k∈ Z
2)2x = (-1)^narcSin(1/2) + nπ, n ∈Z
    2x = (-1)^n*π/6 + nπ, n ∈Z
    x = (-1)^n*π/12 + nπ/2, n ∈Z
3) Sinx = √2/2
    x = (-1)^narcSin√2/2 + nπ, n ∈Z
    x= (-1)^n*π/4 + nπ, n ∈Z
4) Cosx = √2/2
    x = +-arcCos√2/2 +2πk, k ∈Z
    x = +- π/4 + 2πk , k ∈Z