Dặt x=a, y=2b,z=3c

Khi đó

\(P=\frac{yz}{\sqrt{x+yz}}+\frac{xz}{\sqrt{y+xz}}+\frac{xy}{\sqrt{z+xy}}\)và x+y+z=1

Ta có \(\frac{yz}{\sqrt{x+yz}}=\frac{yz}{\sqrt{x\left(x+y+z\right)+yz}}=\frac{yz}{\sqrt{\left(x+y\right)\left(x+z\right)}}\le\frac{1}{2}yz\left(\frac{1}{x+y}+\frac{1}{x+z}\right)\)

=> \(P\le\frac{1}{2}\left(\frac{xz}{x+y}+\frac{yz}{x+y}\right)+\frac{1}{2}\left(\frac{xy}{y+z}+\frac{xz}{y+z}\right)+...=\frac{1}{2}\left(x+y+z\right)\)

                                                                                                                     \(=\frac{1}{2}\)

Vậy \(MaxP=\frac{1}{2}\)khi x=y=z=1/3 hay \(\hept{\begin{cases}a=\frac{1}{3}\\b=\frac{1}{6}\\c=\frac{1}{9}\end{cases}}\)

a: \(a^2+4b^2+9c^2=2ab+6bc+3ac\)

=>\(2a^2+8b^2+18c^2-4ab-12bc-6ac=0\)

=>\(a^2-4ab+4b^2+4b^2-12bc+9c_{}^2+a^2-6ac+9c^2=0\)

=>\(\left(a-2b\right)^2+\left(2b-3c\right)^2+\left(a-3c\right)^2=0\)

=>\(\begin{cases}a-2b=0\\ 2b-3c=0\\ 3c-a=0\end{cases}\Rightarrow a=2b=3c\)

\(A=\left(a-2b+1\right)^{2022}+\left(2b-3c-1\right)^{2023}+\left(3c-a+1\right)^{2024}\)

\(=\left(a-a+1\right)^{2022}+\left(2b-2b-1\right)^{2023}+\left(a-a+1\right)^{2024}\)

=1-1+1

=1

b: \(x^2+2xy+6x+6y+2y^2+8=0\)

=>\(x^2+2xy+y^2+6\left(x+y\right)+9+y^2-1=0\)

=>\(\left(x+y+3\right)^2-1=-y^2\)

=>\(-y^2=\left(x+y+2\right)\left(x+y+4\right)\)

=>\(-y^2=\left(x+y+2024-2022\right)\left(x+y+2024-2020\right)\)

=>\(-y^2=\left(A-2022\right)\left(A-2020\right)\)

mà \(-y^2\le0\forall y\)

nên (A-2022)(A-2020)<=0

=>2020<=A<=2022

\(A_{\min}=2020\) khi x+y+2=0 và y=0

=>y=0 và x=-2-y=-2-0=-2

\(A\max=2022\) khi x+y+4=0 và y=0

=>y=0 và x=-y-4=-4

A.

$a^2+4b^2+9c^2=2ab+6bc+3ac$

$\Leftrightarrow a^2+4b^2+9c^2-2ab-6bc-3ac=0$

$\Leftrightarrow 2a^2+8b^2+18c^2-4ab-12bc-6ac=0$

$\Leftrightarrow (a^2+4b^2-4ab)+(a^2+9c^2-6ac)+(4b^2+9c^2-12bc)=0$

$\Leftrightarrow (a-2b)^2+(a-3c)^2+(2b-3c)^2=0$

$\Rightarrow a-2b=a-3c=2b-3c=0$

$\Rightarrow A=(0+1)^{2022}+(0-1)^{2023}+(0+1)^{2024}=1+(-1)+1=1$

B.

$x^2+2xy+6x+6y+2y^2+8=0$

$\Leftrightarrow (x^2+2xy+y^2)+y^2+6x+6y+8=0$

$\Leftrightarrow (x+y)^2+6(x+y)+9+y^2-1=0$

$\Leftrightarrow (x+y+3)^2=1-y^2\leq 1$ (do $y^2\geq 0$ với mọi $y$)

$\Rightarrow -1\leq x+y+3\leq 1$

$\Rightarrow -4\leq x+y\leq -2$

$\Rightarrow 2020\leq x+y+2024\leq 2022$

$\Rightarrow A_{\min}=2020; A_{\max}=2022$