`cos2 x- 5 sinx + 3 = 0`
`<=>4 - 5 sinx - 2 sin^2x = 0`
`<=>-2 + (5 sinx)/2 + sin^2x = 0`
`<=>25/16 + (5 sinx)/2 + sin^2x = 57/16`
`<=>(sinx + 5/4)^2 = 57/16`
`<=> [(sinx+5/4=sqrt57/4),(sinx+5/4=-sqrt57/4):}`
`<=> [(sinx=sqrt57/4-5/4),(sinx=-sqrt57/4-5/4\text{, loại vì:} -1le sin theta le 1):}`
`<=> [(x=-arcsin(sqrt57/4-5/4)+k2pi),(x=pi+arcsin(sqrt57/4-5/4)+k2pi):}(kinZZ)`
Vậy `S={-arcsin(sqrt57/4-5/4)+k2pi; pi+arcsin(sqrt57/4-5/4)+k2pi | kinZZ}`
`________`
$\\$
`cos6x -3cos3x -1=0`
`<=> -2 - 3 cos3x + 2 cos^2 3x = 0`
`<=> (cos3x - 2) (2 cos3 x+ 1) = 0`
`<=> [(cos3x - 2=0),(2 cos3 x+ 1 = 0):}`
`<=> [(cos3x=2\text{, loại vì:} -1 le cos theta le 1), (cos3x=-1/2):}`
`<=> [(x=(2pi)/3+k2pi),(x=(4pi)/3+k2pi):}(kinZZ)`
Vậy `S={(2pi)/3+k2pi; (4pi)/3+k2pi|kinZZ}`
