`y=cos 2x+cos x`
`<=>y=2cos^2 x-1+cos x`
Đặt `cos x=t (t in [-1;1])`
`=>y=2t^2+t-1`
`y'=4t+1=0<=>t=-1/4`
BBT:
\begin{array}{c|cc} \text{$t$}&\text{$-1$}&\text{}&\text{}\dfrac{-1}{4}&\text{}&\text{}1\\\hline y' & &-&0&+&\\\hline \text{$y$}&\text{}0&\text{}&\text{}&\text{}&2\text{}&\text{}&\\&\text{}&\text{$\searrow$}&\text{}&\text{}\nearrow&\text{}&\text{}\\&\text{$$}&\text{}&\dfrac{-9}{8}\text{}&\text{}&\text{}&\text{}&\text{} \end{array}
`=>{(mi n y=-9/8),(max y=2):}`
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