$b)$ `B=2cos 2x+cos^2 x`

`=2(2cos^2 x-1)+cos^2 x`

`=5cos^2 x-2`

Mà `0<=cos^2 x<=1`

`-> -2<=B=5cos^2 x-2<=3`

`-> B_(Mi n)=-2`

Dấu `=` xảy ra khi `cos x=0` hay `x=(\pi)/(2)+k\pi (k in ZZ)`

$c)$ `C=1-2sin^2 x+cos^2 2x`

`=cos 2x+cos^2 2x`

`=(cos^2 2x+cos 2x+1/4)-1/4`

`=(cos 2x+1/2)^2-1/4>=(-1)/(4) AA x`

`-> C_(Mi n)=(-1)/(4)`

Dấu `=` xảy ra khi `cos 2x=(-1)/(2)` hay `x=+-(\pi)/(3)+k\pi (k in ZZ)`