Ta có [TEX]a+b+c=x+y+z \Rightarrow a-x=y-c+z-b[/TEX]
Lại có [TEX]a+x \geq b+y \geq c+z \geq 0 \Rightarrow y-c \geq z-b \Rightarrow a-x \geq 2(z-b)[/TEX]
Khi đó ta có :
[TEX](ay+bx)-(ac+xz) =a(y-c)-x(z-b) \\\geq a(z-b)-x(z-b) = (a-x)(z-b) \\\geq 2(z-b)^2 \geq 0 \\\Rightarrow ay+bx \geq ac+xz [/TEX]