a/ \(-1\le sin3x\le1\Rightarrow-1\le y\le3\)
\(y_{min}=-1\) khi \(sin3x=-1\)
\(y_{max}=3\) khi \(sin3x=1\)
b/ \(0\le cos^22x\le1\Rightarrow1\le y\le2\)
\(y_{min}=1\) khi \(cos^22x=0\)
\(y_{max}=3\) khi \(cos^22x=1\)
c/ \(y=\sqrt{2}sin\left(x+\frac{\pi}{4}\right)+2\Rightarrow-\sqrt{2}+2\le y\le\sqrt{2}+2\)
\(y_{min}=-\sqrt{2}+2\) khi \(sin\left(x+\frac{\pi}{4}\right)=-1\)
\(y_{max}=\sqrt{2}+2\) khi \(sin\left(x+\frac{\pi}{4}\right)=1\)
d/ \(y=3cosx-\left(2cos^2x-1\right)+5=-2cos^2x+3cosx+6\)
\(y=-2\left(cosx-\frac{3}{4}\right)^2+\frac{57}{8}\le\frac{57}{8}\)
\(y_{max}=\frac{57}{8}\) khi \(cosx=\frac{3}{4}\)
\(y=\left(cosx+1\right)\left(-2cosx+5\right)+1\ge1\)
\(y_{min}=1\) khi \(cosx=-1\)
