`3sin x - 8sin^2 (x/2) + 4 = 0 (1)`
`text{Đặt}` `x/2 = t`
`(1)`
`-> 3sin 2t - 8sin^2 t + 4 = 0`
`-> 3sin 2t - 8.(1 - cos 2t)/(2) + 4 = 0`
`-> 3sin 2t - 4(1 - cos 2t) + 4 = 0`
`-> 3sin 2t - 4 + 4cos 2t + 4 = 0`
`-> 3sin 2t + 4cos 2t = 0`
`-> 3/(5)sin 2t + 4/(5)cos 2t = 0`
`text{Đặt:}`
\(\left\{ \begin{array}{l}\dfrac{3}{5} = cos ∝\\\dfrac{4}{5} = sin ∝\end{array} \right.\)
`-> cos ∝.sin 2t + sin ∝.cos 2t = 0`
`-> sin (2t + ∝) = 0`
`-> 2t + ∝ = kπ`
`-> t = -(∝)/2 + k(π)/2`
`-> 2x = (-∝)/2 + k(π)/2`
`-> x = (-∝)/4 + k(π)/4 (k in ZZ)`
