Question

[dbrby]

cho x>1,y>0 .Cmr: \(\frac{1}{\left(x-1\right)^3}+\frac{\left(x-1\right)^3}{y^3}+\frac{1}{y^3}\ge3\left(\frac{3-2x}{x-1}+\frac{x}{y}\right)\)

Answer 1

[Đinh Đức Hùng]

BĐT\(\Leftrightarrow\left(\frac{1}{x-1}\right)^3+\left(\frac{x-1}{y}\right)^3+\left(\frac{1}{y}\right)^3\ge3\left(\frac{1}{x-1}+\frac{x-1}{y}+\frac{1}{y}-2\right)\)

Đặt \(\left(\frac{1}{x-1};\frac{x-1}{y};\frac{1}{y}\right)=\left(a;b;c\right)\)

BĐT cần cm \(\Leftrightarrow a^3+b^3+c^3\ge3\left(a+b+c-2\right)\)

\(\Leftrightarrow\left(a^3+1+1\right)+\left(b^3+1+1\right)+\left(c^3+1+1\right)\ge3\left(a+b+c\right)\)

Đúng theo AM-GM --> đpcm