`C=(2/(sqrtx+sqrty)+1/(sqrtx-sqrty)+(3sqrtx)/(y-x)):(2-y)/(x-y).đkxđ:xney;x>=0;y>=0`
`=(2/(sqrtx+sqrty)+1/(sqrtx-sqrty)-(3sqrtx)/((sqrtx-sqrty)(sqrtx+sqrty))).((sqrtx-sqrty)(sqrtx+sqrty))/(2-y)`
`=(2sqrtx-2sqrty+sqrtx+sqrty-3sqrtx)/((sqrtx+sqrty)(sqrtx-sqrty)).((sqrtx-sqrty)(sqrtx+sqrty))/(2-y)`
`=(-sqrty)/(2-y)`