Giải thích các bước giải:
` cos2x + 7cosx - sqrt(3)(sin2x - 7sinx) = 8`
`=>(1/2cos2x-sqrt3/2sin2x)+7(1/2cosx+sqrt3/2sinx)=4`
`=>cos(2x+pi/3)+7cos(pi/3-x)=4`
`=>1-2sin^2(x+pi/6)+7sin(x+pi/6)=4`
`=>2sin^2(x+pi/6)-7sin(x+pi/6)+3=0`
`=>`\(\left[ \begin{array}{l}\sin (x+\dfrac{\pi}6)=\dfrac12\\\sin (x+\dfrac{\pi}6)=3\text{ (loại)}\end{array} \right.\)
`=>`\(\left[ \begin{array}{l}x+\dfrac{\pi}6=\dfrac{\pi}6+k2\pi\\x+\dfrac{\pi}6=\dfrac{5\pi}6+k2\pi\end{array} \right.\)`=>`\(\left[ \begin{array}{l}x=k2\pi\\x=\dfrac{2\pi}3+k2\pi\end{array} \right.\)
