d) \(y=4sinx-2cos2x-1\)
\(=4sinx-2\left(1-2sin^2x\right)-1\)
\(=4sin^2x+4sinx-3\)
Đặt \(t=sinx,t\in\left[-1;1\right]\)
\(y=f\left(t\right)=4t^2+4t-3\) \(\Leftrightarrow f'\left(t\right)=8t+4\)
\(f'\left(t\right)=0\Leftrightarrow t=-\dfrac{1}{2}\)
Vẽ BBT với \(t\in\left[-1;1\right]\) ta được
\(minf\left(t\right)=miny=-4\Leftrightarrow t=-\dfrac{1}{2}\)\(\Leftrightarrow sinx=-\dfrac{1}{2}\)\(\Leftrightarrow\left[{}\begin{matrix}x=-\dfrac{\pi}{6}+k2\pi\\x=\dfrac{7\pi}{6}+k2\pi\end{matrix}\right.\) ( k thuộc Z)
\(maxf\left(t\right)=miny=5\Leftrightarrow t=1\)\(\Leftrightarrow sinx=1\) \(\Leftrightarrow x=\dfrac{\pi}{2}+k2\pi\) ( k thuộc Z)
Vậy...
e) \(y=3sin2x+8cos^2x-1\)
\(=3sin2x+4\left(2cos^2x-1\right)+3\)
\(=3sin2x+4cos2x+3\)
\(=5\left(\dfrac{3}{5}sin2x+\dfrac{4}{5}cos2x\right)+3\)
Đặt \(cosu=\dfrac{3}{5}\Leftrightarrow sinu=\dfrac{4}{5}\)
\(y=5\left(sin2x.cosu+cos2x.sinu\right)+3=5.sin\left(2x+u\right)+3\)
Có \(-1\le sin\left(2x+u\right)\le1\) \(\Leftrightarrow-2\le y\le8\)
\(maxy=8\Leftrightarrow sin\left(2x+u\right)=1\) \(\Leftrightarrow2x+u=\dfrac{\pi}{2}+k2\pi\) \(\Leftrightarrow x=-\dfrac{u}{2}+\dfrac{\pi}{4}+k\pi\)\(\Leftrightarrow x=-\dfrac{1}{2}.arccos\dfrac{3}{5}+\dfrac{\pi}{4}+k\pi\) ( k thuộc Z)
\(miny=-2\Leftrightarrow sin\left(2x+u\right)=-1\)\(\Leftrightarrow x=-\dfrac{1}{2}.\dfrac{arccos3}{5}-\dfrac{\pi}{4}+k\pi\) ( k thuộc Z)
Vậy...
f)\(y=4+sin^4x+cos^4x\)
\(=4+\left(sin^2x+cos^2x\right)^2-2sin^2x.cos^2x\)
\(=4+1-\dfrac{1}{2}\left(2sinx.cosx\right)^2\)
\(=5-\dfrac{1}{2}.\left(sin2x\right)^2\)
\(\left(sin2x\right)^2\in\left[0;1\right]\Leftrightarrow y\in\left[\dfrac{9}{2};\dfrac{11}{2}\right]\)
\(maxy=\dfrac{11}{2}\Leftrightarrow sin2x=0\Leftrightarrow2x=k\pi\Leftrightarrow x=\dfrac{k\pi}{2}\) ( k thuộc Z )
\(miny=\dfrac{9}{2}\Leftrightarrow\left(sin2x\right)^2=1\)\(\Leftrightarrow cos2x=0\)\(\Leftrightarrow2x=\dfrac{\pi}{2}+k\pi\)\(\Leftrightarrow x=\dfrac{\pi}{4}+\dfrac{k\pi}{2}\) ( k thuộc Z )
Vậy...
