cos2x=1-2sin²x

y=3+2sin²x-1-3sinx

y=2sin²x-3sinx+2

y=2(sin²x-\(\dfrac{3}{2}\)x+1)

y=2.(sin²x-2.1.\(\dfrac{3}{4}\).sinx+\(\dfrac{9}{16}\)+\(\dfrac{7}{16}\))

y=2.[sin²x-2.1.\(\dfrac{3}{4}\).sinx+(\(\dfrac{3}{4}\))² ]+\(\dfrac{7}{8}\)

y=2.(sinx-\(\dfrac{3}{4}\))²+\(\dfrac{7}{8}\)

Ta có:

-1\(\le\)sinx\(\le\)1

\(\dfrac{-7}{4}\)\(\le\)sinx-\(\dfrac{3}{4}\)\(\le\)1/4

0\(\le\)(sinx-\(\dfrac{3}{4}\))²\(\le\)1/16

0\(\le\)2(sinx-\(\dfrac{3}{4}\))²\(\le\)1/8

7/8\(\le\)2(sinx-\(\dfrac{3}{4}\))²+7/8\(\le\)1

7/8\(\le\)y\(\le\)1

=>min_y=7/8<=>sinx-3/4=0<=>\(\left\{{}\begin{matrix}x=arcsin\dfrac{3}{4}+k2\Pi\\x=\Pi-arcsin\dfrac{3}{4}+k2\Pi\end{matrix}\right.\)

max_y=1<=>sinx=1<=>x=\(\dfrac{\Pi}{2}\)+k2\(\Pi\)

Đặt \(sinx=t\left(t\in\left[-1;1\right]\right)\).

\(\Rightarrow y=f\left(t\right)=-2t^2+3t-1\)

\(\Rightarrow y_{min}=min\left\{f\left(-1\right);f\left(1\right);f\left(\dfrac{3}{4}\right)\right\}=f\left(-1\right)=-6\)

\(y_{max}=max\left\{f\left(-1\right);f\left(1\right);f\left(\dfrac{3}{4}\right)\right\}=f\left(\dfrac{3}{4}\right)=\dfrac{1}{8}\)

\(y=\left|2sin^2x-sinx-1\right|-2sinx\)

Đặt \(sinx=t\in\left[-1;1\right]\)

\(\Rightarrow y=f\left(t\right)=\left|2t^2-t-1\right|-2t\)

BBT cho \(f\left(t\right)\) trên \(\left[-1;1\right]\):

Từ BBT ta thấy \(y_{max}=4\) khi \(sinx=-1\); \(y_{min}=-2\) khi \(sinx=1\)

Đặt \(sinx=t\left(t\in\left[-1;1\right]\right)\)

\(y=\left|sinx+cos2x\right|=\left|2sin^2x-sinx-1\right|\)

\(\Leftrightarrow y=\left|f\left(t\right)\right|=\left|2t^2-t-1\right|\)

\(f\left(-1\right)=2\Rightarrow y=2\)

\(f\left(1\right)=0\Rightarrow y=0\)

\(f\left(\dfrac{1}{4}\right)=-\dfrac{9}{8}\Rightarrow y=\dfrac{9}{8}\)

\(\Rightarrow y_{min}=0;y_{max}=2\)

a, \(y=3-4sin^2x.cos^2x=3-sin^22x\)

Đặt \(sin2x=t\left(t\in\left[-1;1\right]\right)\).

\(\Rightarrow y=f\left(t\right)=3-t^2\)

\(\Rightarrow y_{min}=minf\left(t\right)=2\)

\(y_{max}=maxf\left(t\right)=3\)

b, \(y=f\left(t\right)=\dfrac{-2}{3t-5}\left(t\in\left[0;1\right]\right)\)

\(\Rightarrow y_{min}=minf\left(t\right)=\dfrac{2}{5}\)

\(y_{max}=maxf\left(t\right)=1\)

\(y=sin^3x+2sin^2x+sinx-2\)

đặt \(t=sinx\) với \(t\in\left[-1;1\right]\)

 pt \(\Leftrightarrow\)\(y=t^3+2t^2+t-2\)

\(y'=3t^2+4t+1\)

\(y'=0\Leftrightarrow\left[{}\begin{matrix}t=-1\\t=-\dfrac{1}{3}\end{matrix}\right.\)

vậy GTLN của y = 2 với t=1 \(\Leftrightarrow sinx=1\Leftrightarrow x=\dfrac{\pi}{2}+k2\pi\)

GTNN của y=-58/27  với \(t=-\dfrac{1}{3}\Leftrightarrow sinx=-\dfrac{1}{3}\Leftrightarrow x=sin^{-1}\left(-\dfrac{1}{3}\right)\)

a, \(y=2sin^2x-cos2x=1-2cos2x\)

Vì \(cos2x\in\left[-1;1\right]\Rightarrow y=2sin^2x-cos2x\in\left[-1;3\right]\)

\(\Rightarrow\left\{{}\begin{matrix}y_{min}=-1\\y_{max}=3\end{matrix}\right.\)