8.
\(sinx\in\left[-1;1\right]\Rightarrow\left|sinx\right|\in\left[0;1\right]\)
\(\Rightarrow y=4-3\left|sinx\right|\in\left[1;4\right]\)
\(\Rightarrow\left\{{}\begin{matrix}y_{min}=1\\y_{max}=4\end{matrix}\right.\)
9.
Đặt \(sinx=t\left(t\in\left[-1;1\right]\right)\).
\(\Rightarrow y=f\left(t\right)=t^2-4t+3\)
\(\Rightarrow y_{min}=minf\left(t\right)=min\left\{f\left(-1\right);f\left(1\right)\right\}=0\)
\(y_{max}=maxf\left(t\right)=max\left\{f\left(-1\right);f\left(1\right)\right\}=8\)
Mn giúp mình từ câu 8 đến câu 11 ths ạ,
Cảmon rất nhiềuuu a!
10.
\(y=2sin^2x-cos^2x=-\dfrac{3}{2}cos2x+\dfrac{1}{2}\)
\(cos2x\in\left[-1;1\right]\Rightarrow y=-\dfrac{3}{2}cos2x+\dfrac{1}{2}\in\left[-1;2\right]\)
\(\Rightarrow\left\{{}\begin{matrix}y_{min}=-1\\y_{max}=2\end{matrix}\right.\)
11.
\(y=cos^2x+2cos2x=\dfrac{5}{2}cos2x+\dfrac{1}{2}\)
\(cos2x\in\left[-1;1\right]\Rightarrow y=\dfrac{5}{2}cos2x+\dfrac{1}{2}\in\left[-2;3\right]\)
\(\Rightarrow\left\{{}\begin{matrix}y_{min}=-2\\y_{max}=3\end{matrix}\right.\)
