Bài này à

Gọi thương của phép chia là a thì ta có:

\(x^3+y^3+z^3=a\left(xyz\right)^2\)

Không mất tính tổng quát ta giả sử: \(x\ge y\ge z\)

Dễ thấy \(y^3+z^3⋮x^2\)

\(\Rightarrow y^3+z^3\ge x^2\left(1\right)\)

Ta lại có:

\(3x^3\ge x^3+y^3+z^3=a\left(xyz\right)^2\)

\(\Leftrightarrow3x\ge a\left(yz\right)^2\)

\(\Leftrightarrow9x^2\ge a^2y^4z^4\left(2\right)\)

Từ (1) và (2) suy ra

\(18y^3\ge9\left(y^3+z^3\right)\ge a^2y^4z^4\)

\(\Leftrightarrow z^5\le a^2yz^4\le18\)

\(\Leftrightarrow0< z\le1\)

\(\Leftrightarrow z=1\)

\(\Rightarrow a^2\le a^2y\le18\)

\(\Leftrightarrow1\le a\le4\)

Tự nhiên làm biếng quá thôi còn lại tự làm nốt nha bé.

\(\dfrac{x^3}{y+2z}+\dfrac{y^3}{z+2x}+\dfrac{z^3}{x+2y}=\dfrac{x^4}{xy+2xz}+\dfrac{y^4}{yz+2xy}+\dfrac{z^4}{xz+2yz}\)

\(\ge\dfrac{\left(x^2+y^2+z^2\right)^2}{3\left(xy+yz+zx\right)}\ge\dfrac{\left(x^2+y^2+z^2\right)^2}{3\left(x^2+y^2+z^2\right)}=\dfrac{1}{3}\) 

Dấu "=" xảy ra khi \(x=y=z=\dfrac{1}{\sqrt{3}}\)

ap dung bdt \(x^{m+n}+y^{m+n}\ge x^my^n+x^ny^m\)  (bn tu cm )

\(\Rightarrow x^7+y^7=x^{3+4}+y^{3+4}\ge x^3y^4+x^4y^3\)

\(\Rightarrow\frac{x^2y^2}{x^2y^2+x^7+y^7}\le\frac{x^2y^2}{x^2y^2\left(1+xy^2+x^2y\right)}=\frac{1}{1+x^2y+y^2x}=\frac{1}{xyz+x^2y+y^2x}=\frac{1}{xy\left(x+y+z\right)}=\)

=\(\frac{z}{xyz\left(x+y+z\right)}=\frac{z}{x+y+z}\)

ttu \(P\le\frac{x+y+z}{x+y+z}=1\) đầu = xảy ra khi x=y=z=1

Đặt \(\hept{\begin{cases}2x+y+z=4a\\2y+x+z=4b\\2z+x+y=4c\end{cases}\Rightarrow}\hept{\begin{cases}x=3a-b-c\\y=3b-c-a\\z=3c-a-b\end{cases}}\)thay vào biểu thức đó

\(\Rightarrow\frac{x}{2x+y+z}+\frac{y}{2y+x+z}+\frac{z}{2z+x+y}\)

\(=\frac{3a-b-c}{4a}+\frac{3b-c-a}{4b}+\frac{3c-a-b}{4c}\)

\(=\frac{3}{4}-\frac{b-c}{4a}+\frac{3}{4}-\frac{c-a}{4b}+\frac{3}{4}-\frac{a-b}{4c}\)

\(=\frac{9}{4}-\frac{1}{4}\left(\frac{b}{a}+\frac{c}{a}+\frac{c}{b}+\frac{a}{b}+\frac{a}{c}+\frac{b}{c}\right)\)

Áp dụng BĐT sau: \(\frac{a}{b}+\frac{b}{a}\ge2\Rightarrow\frac{b}{a}+\frac{c}{a}+\frac{c}{b}+\frac{a}{b}+\frac{a}{c}+\frac{b}{c}\ge6\)

\(\Leftrightarrow\frac{1}{4}\left(\frac{b}{a}+\frac{c}{a}+\frac{c}{b}+\frac{a}{b}+\frac{a}{c}+\frac{b}{c}\right)\ge\frac{6}{4}\)

\(\Leftrightarrow\frac{9}{4}-\frac{1}{4}\left(\frac{b}{a}+\frac{c}{a}+\frac{c}{b}+\frac{a}{b}+\frac{a}{c}+\frac{b}{c}\right)\le\frac{3}{4}\)

Từ đó ta có: \(\frac{x}{2x+y+z}+\frac{y}{2y+x+z}+\frac{z}{2z+x+y}\le\frac{3}{4}\)(đpcm).

Dấu "=" xảy ra <=> x=y=z.

Theo Cauchy Schwarz:

\(\frac{x}{2x+y+z}=\frac{x}{\left(x+y\right)+\left(x+z\right)}\le\frac{1}{4}\left(\frac{x}{x+y}+\frac{x}{x+z}\right)\)

Tương tự:

\(\frac{y}{2y+z+x}\le\frac{1}{4}\left(\frac{y}{y+x}+\frac{y}{y+z}\right);\frac{z}{2z+y+x}\le\frac{1}{4}\left(\frac{z}{z+y}+\frac{z}{z+x}\right)\)

Cộng lại:

\(D\le\frac{3}{4}\left(đpcm\right)\)