2cos^2 5x - 2sin 5x + 2 = 0
`<=> 1 + cos^2 5x - sin5x = 0`
`<=> 2 - sin^2 5x- sin5 x = 0`
`<=> -sin^2 5x +sin5x - 2sin5x+2=0`
`<=>(sin5 x - 1) (sin5 x + 2) = 0`
`<=> [(sin5x-1=0),(sin5x+2=0):}`
`<=>[(sin5x=1),(sin5x=-2\ \ \ \ \ \ text{(loại)}):}`
`<=> 5 x = pi/2+k2pi`
Vậy `S={ pi/10+(k2pi)/5|kinZZ}`