`ĐK:` `x,y \geq 0 `
`x ` `\ne` `y `
`P=\sqrt{x}/(\sqrt{x}-\sqrt{y})-(2\sqrt{x})/(\sqrt{x}+\sqrt{y})-(y-3\sqrt{y})/(y-x)`
`=\sqrt{x}/(\sqrt{x}-\sqrt{y})-(2\sqrt{x})/(\sqrt{x}+\sqrt{y})+(y-3\sqrt{y})/[(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})]`
`=[\sqrt{x}(\sqrt{x}+\sqrt{y})-2\sqrt{x}(\sqrt{x}-\sqrt{y})-y+3\sqrt{y}]/[(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})`
`=(x+\sqrt{xy}-2x+2\sqrt{xy}-y+3\sqrt{y})/[(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})]`
`=(-x+3\sqrt{xy}-y+3\sqrt{y})/[(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})]`
Bài 18 :
`a)ĐK` `x \geq 0`
`x \ne 1 `
`M=((\sqrt{x}-2)/(x-1)-(\sqrt{x}+2)/(x+2\sqrt{x}+1)).(x^{2}-2x+1)/2`
`=((\sqrt{x}-2)/((\sqrt{x}-1)(\sqrt{x}+1))-(\sqrt{x}+2)/(\sqrt{x}+1)^{2}).(x-1)^{2}/2`
`=((\sqrt{x}-2)(\sqrt{x}+1)-(\sqrt{x}+2)(\sqrt{x}-1))/((\sqrt{x}-1)(\sqrt{x}+1)^{2}).(x-1)^{2}/2` `=(-2\sqrt{x})/[(\sqrt{x}-1)(\sqrt{x}+1)].(x-1)^{2}/2`
`=[-\sqrt{x}(x-1)^{2}]/[(\sqrt{x}-1)(\sqrt{x}+1)]`
`=-\sqrt{x}(x-1)`
`b)`khi `x=0,16`
`⇔``-\sqrt{0,16}(0,16-1)=-0,4.(-0,84)=0,336`
