ap dung AM-HM ta co :
[tex] \frac{x^2}{a} +\frac{y^2}{b}+\frac{z^2}{c}\geq\frac{x+y+z}{a+b+c} =\frac{1}{a+b+c}[/tex]
"=" <=> [tex] \frac{x}{a}=\frac{y}{b}=\frac{z}{c}[/tex]
ta co : [tex] \frac{x}{a}=\frac{y}{b}=\frac{z}{c}=\frac{x+y+z}{a+b+c}= \frac{1}{a+b+c}[/tex]
=> [tex]A=(x+y+z).\frac{x^{2005}}{a^{2005}}=(\frac{x}{a})^{2005} =\frac{1}{(a+b+c)^{2005}}[/tex]