a)`((\sqrt{x}-2)/(x-1)-(\sqrt{x}+2)/(x+2\sqrt{x}+1)).(1-x)^{2}/2`
`=((\sqrt{x}-2)/((\sqrt{x}-1)(\sqrt{x}+1))-(\sqrt{x}+2)/(\sqrt{x}+1)^{2}).(1-x)^{2}/2`
`=((\sqrt{x}-2)(\sqrt{x}+1)-(\sqrt{x}+2)(x-1))/((\sqrt{x}-1)(\sqrt{x}+1)^{2}).(1-x)^{2}/2`
`=(x-\sqrt{x}-2-x-\sqrt{x}+2)/((x-1)(\sqrt{x}+1)).((x-1)(\sqrt{x}+1)(\sqrt{x}-1))/2`
`=(-2\sqrt{x}(\sqrt{x}-1))/2`
`=-\sqrt{x}(1-\sqrt{x})`
`=x-\sqrt{x}`
b) Để A dương
`->x-\sqrt{x}>0`
`->\sqrt{x}<1`
`->0<x<1`
Vậy `0<x<1` thì `A` dương
