Question

[Nguyễn Linh Chi]

Tìm min, max của :

1. y = \(\sqrt{4-sin^52x}-8\)

2. y = \(\dfrac{4}{\sqrt{5-2cos^2x.sin^2x}}\)

Answer 1

[Lê Thị Thục Hiền]

2.Biểu thức luôn xác định

\(y=\dfrac{4}{\sqrt{5-2cos^2sin^2x}}=\dfrac{4}{\sqrt{5-\dfrac{1}{2}sin^22x}}\)

Có: \(1\ge sin^22x\ge0\)

\(\Leftrightarrow-\dfrac{1}{2}\le-\dfrac{1}{2}sin^22x\le0\)

\(\Leftrightarrow\dfrac{3\sqrt{2}}{2}\le\sqrt{5-\dfrac{1}{2}sin^22x}\le\sqrt{5}\)

\(\Rightarrow\dfrac{4\sqrt{2}}{3}\ge y\ge\dfrac{4\sqrt{5}}{5}\)

miny=\(\dfrac{4\sqrt{5}}{5}\) \(\Leftrightarrow sin2x=0\)\(\Leftrightarrow x=\dfrac{k\pi}{2}\left(k\in Z\right)\)

maxy=\(\dfrac{4\sqrt{2}}{3}\Leftrightarrow\left[{}\begin{matrix}sin2x=1\\sin2x=-1\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{\pi}{4}+k\pi\\x=\dfrac{-\pi}{4}+k\pi\end{matrix}\right.\)\(\left(k\in Z\right)\)

Answer 2

[Lê Thị Thục Hiền]

1.Biểu thức luôn xác định

Xét \(sin2x=0\) \(\Leftrightarrow x=\dfrac{k\pi}{2}\left(k\in Z\right)\) khi đó \(y=-6\)

Xét \(sin2x\ne0\) 

=> \(1\ge sin^52x\ge-1\)

\(\Leftrightarrow4-1\le4-sin^52x\le4+1\)

\(\Leftrightarrow\sqrt{3}\le\sqrt{4-sin^52x}\le\sqrt{5}\)

\(\Leftrightarrow\sqrt{3}-8\le y\le\sqrt{5}-8\)

\(y=\sqrt{3}-8< -6\) , \(y=\sqrt{5}-8>-6\)

=>min= \(\sqrt{3}-8\) \(\Leftrightarrow sin2x=1\left(tm\right)\) \(\Leftrightarrow x=\dfrac{\pi}{4}+k\pi\left(k\in Z\right)\)

maxy=\(\sqrt{5}-8\)\(\Leftrightarrow sin2x=-1\left(tm\right)\) \(\Leftrightarrow x=-\dfrac{\pi}{4}+k\pi\left(k\in Z\right)\)

(câu này e ko chắc)