Câu 1:

`a)` Đúng

`f'(x)=-2sin2x+1=0<=>sin2x=1/2`

`<=>sin2x=sin` `pi/6`
`<=>`\(\left[ \begin{array}{l}2x=\frac{\pi}{6}+k2\pi\\2x=\pi-\frac{\pi}{6}+k2\pi\end{array} \right.\)  

`<=>`\(\left[ \begin{array}{l}x=\frac{\pi}{12}+k\pi\\x=\frac{5\pi}{12}+k\pi\end{array} \right.\)  

`x in[0;pi]=>`\(\left[ \begin{array}{l}x=\frac{\pi}{12}\\x=\frac{5\pi}{12}\end{array} \right.\)  

`f(x)=cos2x+x`

`=>f(0)=1`

`=>f(pi/12)=sqrt3/2+pi/12~~1,128`

`=>f((5pi)/12)=-sqrt3/2+(5pi)/12~~0,443` (GTNN)

`f(pi)=1+pi~~4,14`
`b)` Sai

`f'(x)=-2sin2x+1`

`c)` Đúng (theo câu a)

`d)` Sai (theo câu a)

 `x in[0;pi]=>`\(\left[ \begin{array}{l}x=\frac{\pi}{12}\\x=\frac{5\pi}{12}\end{array} \right.\)