giúp mink với ạ

`a,((\sqrt{x}+\sqrt{y})/(\sqrt{x}-\sqrt{y}) - (\sqrt{x}-\sqrt{y})/(\sqrt{x}+\sqrt{y})) : \sqrt{xy}/(x-y)(x;y\ne 0 ; x\ne y)`
`=((\sqrt{x}+\sqrt{y})^2 - (\sqrt{x}-\sqrt{y})^2)/((\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})) . (x-y)/\sqrt{xy}`
`=(x+2\sqrt{xy} + y - x + 2\sqrt{xy} - y)/((\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})) . (x-y)/\sqrt{xy}`
`=(4\sqrt{xy}(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y}))/(\sqrt{xy}(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y}))`
`=4(đpcm)`
`b,(1 + (a+\sqrt{a})/(\sqrt{a} + 1)) . (1 - (a-\sqrt{a})/(\sqrt{a}-1)) (a>0;a\ne 1 )`
`=(a + 2\sqrt{a} + 1)/(\sqrt{a} + 1) . -(a-2\sqrt{a} +1)/(\sqrt{a}-1)`
`=-(\sqrt{a} + 1)(\sqrt{a}-1)`
`=-(a-1) = 1 -a(đpcm)`