`cos2x+3sinx-2=0`
`<=> -1 + 3 sinx - 2 sin^2x = 0`
`<=> -(sinx - 1) (2 sinx - 1) = 0`
`<=> [{:( sin x - 1 = 0),(2 sin x - 1 = 0):}:}`
`<=> [{:( sin x = 1),(sin x = 1/2):}:}`
`<=> [{:( x = pi/2+k2pi),(x = pi/6+k2pi),(x=[5pi]/6+k2pi):}:} (kinZZ)`
$\\$
`cos2x+cosx-2=0`
`<=> -3 + cosx + 2 cos^2x = 0`
`<=> (cosx - 1) (2 cosx + 3) = 0`
`<=> [{:( cos x - 1 = 0),(2 cos x + 3 = 0):}:}`
`<=> [{:( cos x = 1),(cos x = -3/2 \text{, loại do} -1lesinθle1):}:}`
`<=> x=k2pi \ (kinZZ)`
