\(x,y,z>0\)

Áp dụng BĐT Caushy cho 3 số ta có:

\(x^3+y^3+z^3\ge3\sqrt[3]{x^3y^3z^3}=3xyz\ge3.1=3\)

\(P=\dfrac{x^3-1}{x^2+y+z}+\dfrac{y^3-1}{x+y^2+z}+\dfrac{z^3-1}{x+y+z^2}\)

\(=\dfrac{\left(x^3-1\right)^2}{\left(x^2+y+z\right)\left(x^3-1\right)}+\dfrac{\left(y^3-1\right)^2}{\left(x+y^2+z\right)\left(y^3-1\right)}+\dfrac{\left(z^3-1\right)^2}{\left(x+y+z^2\right)\left(x^3-1\right)}\)

Áp dụng BĐT Caushy-Schwarz ta có:

\(P\ge\dfrac{\left(x^3+y^3+z^3-3\right)^2}{\left(x^2+y+z\right)\left(x^3-1\right)+\left(x+y^2+z\right)\left(y^3-1\right)+\left(x+y^2+z\right)\left(y^3-1\right)}\)

\(\ge\dfrac{\left(3-3\right)^2}{\left(x^2+y+z\right)\left(x^3-1\right)+\left(x+y^2+z\right)\left(y^3-1\right)+\left(x+y^2+z\right)\left(y^3-1\right)}=0\)

\(P=0\Leftrightarrow x=y=z=1\)

Vậy \(P_{min}=0\)

Bổ đề:\(\dfrac{1}{x}+\dfrac{1}{y}\ge\dfrac{4}{x+y}\Leftrightarrow\dfrac{1}{x+y}\le\dfrac{1}{4}\left(\dfrac{1}{x}+\dfrac{1}{y}\right)\)

Ta có:\(\dfrac{1}{2x+y+z}\le\dfrac{1}{4}\left(\dfrac{1}{x+y}+\dfrac{1}{x+z}\right)\le\dfrac{1}{4}.\dfrac{1}{4}\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{x}+\dfrac{1}{z}\right)\)

Tương tự ta có:\(\dfrac{1}{2y+z+x}\le\dfrac{1}{4}.\dfrac{1}{4}\left(\dfrac{1}{y}+\dfrac{1}{z}+\dfrac{1}{y}+\dfrac{1}{x}\right)\)

                         \(\dfrac{1}{2z+x+y}\le\dfrac{1}{4}.\dfrac{1}{4}\left(\dfrac{1}{z}+\dfrac{1}{x}+\dfrac{1}{z}+\dfrac{1}{y}\right)\)

Cộng vế với vế ta có:

\(\dfrac{1}{2x+y+z}+\dfrac{1}{2y+z+x}+\dfrac{1}{2z+x+y}\le\dfrac{1}{16}\left[4\left(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\right)\right]=\dfrac{1}{16}.4.4=1\)

Dấu "=" xảy ra ⇔ \(x=y=z=\dfrac{3}{4}\)

ai lm dc bài này ko ạ. mik đang cần lắm

giả sử cả 3 số xyz đều nhỏ hơn 1 

=>x+y+z<1+1+1=3

ta có x+y+z>\(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}\)=\(\dfrac{xy+yz+xz}{xyz}\)\(\ge\)\(\dfrac{3\sqrt[3]{\left(abc\right)^2}}{abc}\) =\(\dfrac{3}{\sqrt[3]{abc}}=\dfrac{3}{\sqrt[3]{1}}=3\) vậy x+y+z >3

từ đó sẽ có ít nhất 1 trong 3 số lớn hơn 1

Lời giải:

Ta có:

\(A=\frac{1}{x^2+x}+\frac{1}{y^2+y}+\frac{1}{z^2+z}=\frac{1}{x(x+1)}+\frac{1}{y(y+1)}+\frac{1}{z(z+1)}\)

\(=\frac{1}{x}-\frac{1}{x+1}+\frac{1}{y}-\frac{1}{y+1}+\frac{1}{z}-\frac{1}{z+1}\)

\(=\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\right)-\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)(1)\)

Áp dụng BĐT Cauchy-Schwarz:

\(\frac{1}{x}+\frac{1}{1}\geq \frac{4}{x+1}\) và tương tự với các phân thức còn lại rồi cộng lại:

\(\Rightarrow \frac{1}{x}+\frac{1}{y}+\frac{1}{z}+3\geq 4\left(\frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\right)\)

\(\Leftrightarrow \frac{1}{x+1}+\frac{1}{y+1}+\frac{1}{z+1}\leq \frac{1}{4}\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}+3\right)(2)\)

Từ (1); (2) suy ra \(A\geq \frac{3}{4}\left(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}-1\right)\)

Mà theo BĐT Cauchy- Schwarz ta có:

\(\frac{1}{x}+\frac{1}{y}+\frac{1}{z}\geq \frac{9}{x+y+z}=\frac{9}{3}=3\)

Do đó: \(A\geq \frac{3}{4}(3-1)=\frac{3}{2}\) (đpcm)

Dấu bằng xảy ra khi \(x=y=z=1\)

do x,y,z là các số dương nên

\(x^2-xy+y^2\ge xy\Leftrightarrow x^3+y^3\ge xy\left(x+y\right)\)

tương tự ta cũng có : \(y^3+z^3\ge yz\left(y+z\right)\)

\(z^3+x^3\ge zx\left(z+x\right)\)

\(\Rightarrow\Sigma\dfrac{1}{x^3+y^3+xyz}\le\Sigma\dfrac{1}{xy\left(x+y+z\right)}=\dfrac{1}{x+y+z}\left(\dfrac{1}{xy}+\dfrac{1}{yz}+\dfrac{1}{xz}\right)\)

\(=\dfrac{1}{x+y+z}\left(\dfrac{x+y+z}{xyz}\right)=\dfrac{1}{xyz}\left(đpcm\right)\)