`1)(\sqrtx)/(\sqrtx+3)+(2\sqrtx)/(\sqrtx-3)-(3x+9)/(x-9)`

`-` ĐKXĐ : `x>=0;x\ne9`

`=(\sqrtx)/(\sqrtx+3)+(2\sqrtx)/(\sqrtx-3)-(3x+9)/((\sqrtx-3)(\sqrtx+3))`

`=(\sqrtx(\sqrtx-3))/((\sqrtx+3)(\sqrtx-3))+(2\sqrtx(\sqrtx+3))/((\sqrtx+3)(\sqrtx-3))-(3x+9)/((\sqrtx-3)(\sqrtx+3))`

`=(\sqrtx(\sqrtx-3)+2\sqrtx(\sqrtx+3)-(3x+9))/((\sqrtx-3)(\sqrtx+3))`

`=(x-3\sqrtx+2x+6\sqrtx-3x-9)/((\sqrtx-3)(\sqrtx+3))`

`=(3\sqrtx-9)/((\sqrtx-3)(\sqrtx+3))`

`=(3(\sqrtx-3))/((\sqrtx-3)(\sqrtx+3))`

`=3/(\sqrtx+3)`

`2)((\sqrta)/(\sqrta+1)-(\sqrta)/(a+\sqrta)):(\sqrta-1)/(a-1)`

`-` ĐKXĐ : `a>=0;a\ne1`

`=[(\sqrta)/(\sqrta+1)-(\sqrta)/(\sqrta(\sqrta+1))]:(\sqrta-1)/((\sqrta-1)(\sqrta+1))`

`=[(\sqrta.\sqrta)/(\sqrta(\sqrta+1))-(\sqrta)/(\sqrta(\sqrta+1))]:(1)/(\sqrta+1)`

`=[(a)/(\sqrta(\sqrta+1))-(\sqrta)/(\sqrta(\sqrta+1))]:(1)/(\sqrta+1)`

`=[(a-\sqrta)/(\sqrta(\sqrta+1))]:(1)/(\sqrta+1)`

`=(\sqrta(\sqrta-1))/(\sqrta(\sqrta+1)).(\sqrta+1)`

`=(\sqrta-1)/(\sqrta+1).(\sqrta+1)`

`=\sqrta-1`

`@` ltmn