Cho \(xy=1\)và \(x,y>0\)

Tìm \(M_{max}=\frac{x}{x^4+y^2}+\frac{y}{x^2+y^4}\)

\(M=\frac{x}{x^4+\frac{1}{x^2}}+\frac{x}{y^2+\frac{1}{y^2}}\)

\(M=\frac{x^4}{x^6+1}+\frac{y^3}{y^6+1}\)

Áp dụng BĐT Cauchy

\(x^6+1\ge2x^3=>\frac{x^2}{x^6+1}\le\frac{1}{2}\)

Tương tự \(\frac{y^3}{y^6+1}\le\frac{1}{2}\)

\(=>M\le1\)

Dấu "=" xảy ra khi \(\hept{\begin{cases}xy=1\\x=1\\y=1\end{cases}}\Leftrightarrow x=y=1\)

Vậy \(M_{max}=1\)khi \(x=y=1\)

với x,y dương, áp dụng bđt cosi ta có:

 \(x^4+y^2\ge2\sqrt{x^4.y^2}=2x.xy=2x\left(xy=1\right)\Rightarrow\frac{x}{x^4+y^2}\le\frac{x}{2x}=\frac{1}{2}\)

tương tự thì: \(\frac{y}{x^2+y^4}\le\frac{1}{2}\)

=> (gọi là A đi ): \(A\le\frac{1}{2}+\frac{1}{2}=1\Leftrightarrow x=y=1\)

1. \(1=x^2+y^2\ge2xy\Rightarrow xy\le\frac{1}{2}\)

 \(A=-2+\frac{2}{1+xy}\ge-2+\frac{2}{1+\frac{1}{2}}=-\frac{2}{3}\)

max A = -2/3 khi x=y=\(\frac{\sqrt{2}}{2}\)

\(\frac{1}{xy}+\frac{1}{xz}=\frac{1}{x}\left(\frac{1}{y}+\frac{1}{z}\right)\ge\frac{1}{x}.\frac{4}{y+z}=\frac{4}{\left(4-t\right)t}=\frac{4}{4-\left(t-2\right)^2}\ge1\) với t = y+z => x =4 -t

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

\(x^2+\frac{1}{x^2}\ge2.\sqrt{x^2.\frac{1}{x^2}}=2\)

\(x^2+\frac{y^2}{4}\ge2.\sqrt{x^2.\frac{y^2}{4}}=2.\left|\frac{xy}{2}\right|=\left|xy\right|\)

=> \(4=\left(x^2+\frac{1}{x^2}\right)+\left(x^2+\frac{y^2}{4}\right)\ge2+\left|xy\right|\)

=> \(\left|xy\right|\le2\Rightarrow xy\le2\)

Vậy Max (xy) = 2 khi |x| = 1 và |y| = 2.|x| = 2

\(P\le\frac{1}{2}\left(\Sigma\frac{1}{\sqrt{xy}}\right)\le\frac{\left(xy+yz+zx\right)^2}{6x^2y^2z^2}\le\frac{\left(x^2+y^2+z^2\right)^2}{6x^2y^2z^2}=\frac{3}{2}\)

dấu "=" xảy ra \(\Leftrightarrow\)\(x=y=z=1\)

mình nhầm :) làm lại nhé

\(P\le\frac{1}{2}\left(\Sigma\frac{1}{\sqrt{xy}}\right)\le\frac{\left(\sqrt{xy}+\sqrt{yz}+\sqrt{zx}\right)^2}{6xyz}\le\frac{xy+yz+zx}{2xyz}\le\frac{x^2+y^2+z^2}{2xyz}=\frac{3}{2}\)

\(P\le\frac{x}{2\sqrt{x^4.y^2}}+\frac{y}{2\sqrt{x^2.y^4}}=\frac{x}{2x^2y}+\frac{y}{2xy^2}=\frac{1}{2xy}+\frac{1}{2xy}=\frac{1}{xy}=1\)

Dấu "=" xảy ra khi x=y=1

Ta có :\(2x^2+\frac{1}{x^2}+\frac{y^2}{4}=4\Leftrightarrow\left(x^2+\frac{1}{x^2}-2\right)+\left(x^2+\frac{y^2}{4}-xy\right)+xy=2\)

\(\Leftrightarrow\left(x-\frac{1}{x}\right)^2+\left(x-\frac{y}{2}\right)^2=2-xy\)

\(\Rightarrow2-xy\ge0\Leftrightarrow xy\le2\) có GTLN là \(2\)

Dấu "=" xảy ra \(\Leftrightarrow x=1;y=2\)