Lời giải:

a. $y=\sqrt{x^2+x-2}\geq 0$ (tính chất cbh số học)

Vậy $y_{\min}=0$. Giá trị này đạt tại $x^2+x-2=0\Leftrightarrow x=1$ hoặc $x=-2$
b.

$y^2=6+2\sqrt{(2+x)(4-x)}\geq 6$ do $2\sqrt{(2+x)(4-x)}\geq 0$ theo tính chất căn bậc hai số học

$\Rightarrow y\geq \sqrt{6}$ (do $y$ không âm)

Vậy $y_{\min}=\sqrt{6}$ khi $x=-2$ hoặc $x=4$

$y^2=(\sqrt{2+x}+\sqrt{4-x})^2\leq (2+x+4-x)(1+1)=12$ theo BĐT Bunhiacopxky

$\Rightarrow y\leq \sqrt{12}=2\sqrt{3}$

Vậy $y_{\max}=2\sqrt{3}$ khi $2+x=4-x\Leftrightarrow x=1$

c. ĐKXĐ: $-2\leq x\leq 2$

$y^2=(x+\sqrt{4-x^2})^2\leq (x^2+4-x^2)(1+1)$ theo BĐT Bunhiacopxky

$\Leftrightarrow y^2\leq 8$

$\Leftrightarrow y\leq 2\sqrt{2}$

Vậy $y_{\max}=2\sqrt{2}$ khi $x=\sqrt{2}$

Mặt khác:

$x\geq -2$

$\sqrt{4-x^2}\geq 0$

$\Rightarrow y\geq -2$
Vậy $y_{\min}=-2$ khi $x=-2$

a: \(5-2\cdot cos^2x\cdot\sin^2x\)

\(=5-2\cdot\left(\sin x\cdot cosx\right)^2\)

\(=5-2\cdot\left\lbrack\frac12\cdot2\cdot\sin x\cdot cosx\right\rbrack^2=5-2\cdot\left\lbrack\frac12\cdot\sin2x\right\rbrack^2\)

\(=5-2\cdot\frac14\cdot\sin^22x=-\frac12\cdot\sin^22x+5\)

\(0\le\sin^22x\le1\)

=>\(0\ge-\frac12\sin^22x\ge-\frac12\)

=>\(0+5\ge-\frac12\sin^22x+5\ge-\frac12+5\)

=>\(5\ge-\frac12\sin^22x+5\ge\frac92\)

=>\(\frac92\le-\frac12\sin^22x+5\le5\)

=>\(\sqrt{\frac92}\le\sqrt{-\frac12\cdot\sin^22x+5}\le\sqrt5\)

=>\(\frac{3\sqrt2}{2}\le\sqrt{-\frac12\cdot\sin^22x+5}\le\sqrt5\)

=>\(\frac{2}{3\sqrt2}\ge\frac{1}{\sqrt{-\frac12\cdot\sin^22x+5}}\ge\frac{1}{\sqrt5}\)

=>\(\frac{2\cdot4}{3\sqrt2}\ge\frac{1\cdot4}{\sqrt{-\frac12\cdot\sin^22x+5}}\ge\frac{1\cdot4}{\sqrt5}\)

=>\(\frac{4\sqrt2}{3}\ge y\ge\frac{4}{\sqrt5}\)

=>\(y_{\max}=\frac{4\sqrt2}{3}\) khi \(-\frac12\cdot\sin^22x+5=\frac92\)

=>\(-\frac12\cdot\sin^22x=-\frac12\)

=>\(\sin^22x=1\)

=>\(cos^22x=0\)

=>cos2x=0

=>\(2x=\frac{\pi}{2}+k\pi\)

=>\(x=\frac{\pi}{4}+\frac{k\pi}{2}\)

\(y_{\min}=\frac{4}{\sqrt5}\) khi \(-\frac12\cdot\sin^22x+5=5\)

=>\(\sin^22x=0\)

=>sin 2x=0

=>\(2x=k\pi\)

=>\(x=\frac{k\pi}{2}\)

b: \(f\left(x\right)=3\cdot\sin^2x+5\cdot cos^2x-4\cdot cos2x-2\)

\(=3\left(1-cos^2x\right)+5\cdot cos^2x-4\left(2\cdot cos^2x-1\right)-2\)

\(=3-3\cdot cos^2x+5\cdot cos^2x-8\cdot cos^2x+4-2=-6\cdot cos^2x+5\)

Ta có: \(0<=cos^2x\le1\)

=>\(0\ge-6\cdot cos^2x\ge-6\)

=>\(0+5\ge-6\cdot cos^2x+5\ge-6+5\)

=>5>=y>=-1

Do đó: \(y_{\min}=-1\) khi \(-6\cdot cos^2x+5=-1\)

=>\(-6\cdot cos^2x=-6\)

=>\(cos^2x=1\)

=>\(\sin^2x=0\)

=>sin x=0

=>\(x=k\pi\)

y max=5 khi \(-6\cdot cos^2x+5=5\)

=>\(-6\cdot cos^2x=0\)

=>cosx=0

=>\(x=\frac{\pi}{2}+k\pi\)

a: \(5-2\cdot cos^2x\cdot\sin^2x\)

\(=5-2\cdot\left(\sin x\cdot cosx\right)^2\)

\(=5-2\cdot\left(\frac12\cdot\sin2x\right)^2=5-2\cdot\frac14\cdot\sin^22x=-\frac12\cdot\sin^22x+5\)

Ta có: \(0\le\sin^22x\le1\)

=>\(-\frac12\le-\frac12\cdot\sin^22x\le0\)

=>\(-\frac12+5\le-\frac12\cdot\sin^22x+5\le0+5\)

=>\(\frac92\le-\frac12\cdot\sin^22x+5\le5\)

=>\(\frac{3\sqrt2}{2}\le\sqrt{-\frac12\cdot\sin^22x+5}\le\sqrt5\)

=>\(4:\frac{3\sqrt2}{2}\ge\frac{4}{\sqrt{-\frac12\cdot sin^22x+5}}\ge\frac{4}{\sqrt5}\)

=>\(\frac{2\sqrt2}{3}\ge y\ge\frac{4\sqrt5}{5}\)

Do đó: \(y_{\max}=\frac{2\sqrt2}{3}\) khi \(\sin^22x=1\)

=>\(cos^22x=0\)

=>cos2x=0

=>\(2x=\frac{\pi}{2}+k\pi\)

=>\(x=\frac{\pi}{4}+\frac{k\pi}{2}\)

\(y_{\min}=\frac{4\sqrt5}{5}\) khi \(\sin^22x=0\)

=>sin 2x=0

=>\(2x=k\pi\)

=>\(x=\frac{k\pi}{2}\)

b: \(f\left(x\right)=3\cdot\sin^2x+5\cdot cos^2x-4\cdot cos2x-2\)

\(=3\cdot\sin^2x+5\cdot cos^2x-4\left(cos^2x-\sin^2x\right)-2\)

\(=3\cdot\sin^2x+5\cdot cos^2x-4\cdot cos^2x+4\cdot\sin^2x-2\)

\(=7\cdot\sin^2x+cos^2x-2=7\cdot\sin^2x+1-\sin^2x-2=6\cdot\sin^2x-1\)

Ta có: \(0\le\sin^2x\le1\)

=>\(0\le6\sin^2x\le6\)

=>\(0-1\le6\sin^2x-1\le6-1\)

=>-1<=f(x)<=5

f(x) min=-1 khi \(\sin^2x=0\)

=>sin x=0

=>\(x=k\pi\)

f(x) max=5 khi \(\sin^2x=1\)

=>\(cos^2x=0\)

=>cosx=0

=>\(x=\frac{\pi}{2}+k\pi\)

\(x=\sqrt[3]{9+4\sqrt5}+\sqrt[3]{9-4\sqrt5}\)

=>\(x^3=9+4\sqrt5+9-4\sqrt5+3\cdot x\cdot\sqrt[3]{\left(9-4\sqrt5\right)\left(9+4\sqrt5\right)}\)

=>\(x^3=3x+18\)

=>\(x^3-3x=18\)

\(y=\sqrt[3]{3+2\sqrt2}+\sqrt[3]{3-2\sqrt2}\)

=>\(y^3=3+2\sqrt2+3-2\sqrt2+3\cdot\sqrt[3]{\left(3+2\sqrt2\right)\left(3-2\sqrt2\right)}\left(\sqrt[3]{3+2\sqrt2}+\sqrt[3]{3-2\sqrt2}\right)\)

=>\(y^3=6+3y\)

=>\(y^3-3y=6\)

\(P=x^3+y^3-3\left(x+y\right)+1969\)

\(=\left(x^3-3x\right)+\left(y^3-3y\right)+1969\)

=18+6+1969

=1969+24

=1993

Áp dụng BĐT cô-si,ta có:

\(\dfrac{x}{\sqrt{y}}+\dfrac{y}{\sqrt{x}}\)>=2 căn bậc 4 của x.y

Vậy A>= -căn x-căn y - căn 4 của x.y=-(\(\sqrt{x}-\sqrt{y}\) )^2

Mà (\(\left(\sqrt{x}-\sqrt{y}\right)^2>=0\)

Suy ra A<=0

dấu = xảy ra khi và chỉ khi x=y

Đặt \(\left\{{}\begin{matrix}x=sina\\y=sinb\end{matrix}\right.\) với \(a;b\in\left(0;\dfrac{\pi}{2}\right)\)

\(P=\sqrt{sina}+\sqrt{sinb}+\sqrt[4]{12}.\sqrt{sina.cosb+cosa.sinb}\)

\(P\le\sqrt{2\left(sina+sinb\right)}+\sqrt[4]{12}.\sqrt{sin\left(a+b\right)}\)

Do \(sina+sinb=2sin\dfrac{a+b}{2}cos\dfrac{a-b}{2}\le2sin\dfrac{a+b}{2}\)

\(\Rightarrow P\le2\sqrt{sin\dfrac{a+b}{2}}+\sqrt[4]{12}.\sqrt{sin\left(a+b\right)}=2\sqrt{sint}+\sqrt[4]{12}.\sqrt{sin2t}\)

\(\Rightarrow\dfrac{P}{\sqrt{2}}\le\sqrt{2sint}+\sqrt{\sqrt{3}.sin2t}\Rightarrow\dfrac{P^2}{4}\le2sint+\sqrt{3}sin2t\)

\(\Rightarrow\dfrac{P^2}{8}\le sint\left(1+\sqrt{3}cost\right)\Rightarrow\dfrac{P^4}{64}\le sin^2t\left(1+\sqrt{3}cost\right)^2\le2sin^2t\left(1+3cos^2t\right)\)

\(\Leftrightarrow\dfrac{P^4}{128}\le sin^2t\left(4-3sin^2t\right)=-3sin^4t+4sin^2t\)

\(\Leftrightarrow\dfrac{P^4}{128}\le-3\left(sin^2t-\dfrac{2}{3}\right)^2+\dfrac{4}{3}\le\dfrac{4}{3}\)

\(\Rightarrow P\le4.\sqrt[4]{\dfrac{2}{3}}\)

Dấu "=" xảy ra khi và chỉ khi \(sint=\sqrt{\dfrac{2}{3}}\)