Question

Cho x,y,z>0 thỏa mãn x\(^3\)+y\(^3\)+z\(^3\)=1

CMR: \(\dfrac{x^2}{\sqrt{1-x^2}}\)+\(\dfrac{y^2}{\sqrt{1-y^2}}\)+\(\dfrac{z^2}{\sqrt{1-z^2}}\)>=2

Answer 1

Áp dụng BĐT Cauchy , ta có :

\(\dfrac{x^2}{\sqrt{1-x^2}}=\dfrac{x^3}{x\sqrt{1-x^2}}\ge\dfrac{x^3}{\dfrac{x^2+1-x^2}{2}}=2x^3\)

\(\dfrac{y^2}{\sqrt{1-y^2}}=\dfrac{y^3}{y\sqrt{1-y^2}}\ge\dfrac{y^3}{\dfrac{y^2+1-y^2}{2}}=2y^3\)

\(\dfrac{z^2}{\sqrt{1-z^2}}=\dfrac{z^3}{z\sqrt{1-z^2}}\ge\dfrac{z^3}{\dfrac{z^2+1-z^2}{2}}=2z^3\)

\(\Rightarrow\dfrac{x^2}{\sqrt{1-x^2}}+\dfrac{y^2}{\sqrt{1-y^2}}+\dfrac{z^2}{\sqrt{1-z^2}}\ge2\left(x^3+y^3+z^3\right)=2\)