`D = ((\sqrtx)/(3+\sqrtx) + (x-9)/(9-x)) :( ( 3\sqrtx+1)/(x-3\sqrtx) - 1/\sqrtx)`
`= (\sqrtx (3-\sqrtx) + x-9)/((3+\sqrtx)(3-\sqrtx)): (3\sqrtx+1-1(\sqrtx-3))/((\sqrtx(\sqrtx-3))`
`= (3\sqrtx-x+x-9)/(9-x) : (3\sqrtx+1-\sqrtx+3)/(x-3\sqrtx)`
`= (3\sqrtx-9)/(9-x) : (2\sqrtx+4)/(x-3\sqrtx)`
`= (-3(\sqrtx-3))/((\sqrtx-3)(\sqrtx+3)) . (\sqrtx(\sqrtx-3))/(2\sqrtx+4)`
= `(-3\sqrtx(\sqrt-3))/((\sqrtx+3)(2\sqrtx+4))`
`D < -1 <=> (-3\sqrtx(\sqrtx-3))/((\sqrtx+3)(2\sqrtx+4)) < -1 <=>....`
lát giải tiếp :v
