a. `2019-\sqrt{1-4x+4x^{2}}=0`
`->2019-\sqrt{(1-2x)^{2}}=0`
`->2019-|1-2x|=0`
`TH1:xle1/2`
`->2019-1+2x=0`
`->2x=-2018`
`->x=-1009``(TM)`
`TH2:x>1/2`
`->2019-2x+1=0`
`->-2x=-2020`
`->x=1010``(TM)`
Vậy `S={-1009;1010}`
b. `T=((\sqrt{x}+2)/(x-9)-(\sqrt{x}-2)/(x+6\sqrt{x}+9)).(\sqrt{x}-9/\sqrt{x})`
`=((\sqrt{x}+2)/((\sqrt{x}-3)(\sqrt{x}+3))-(\sqrt{x}-2)/(\sqrt{x}+3)^{2}).(x-9)/\sqrt{x}`
`=((\sqrt{x}+2)(\sqrt{x}+3)-(\sqrt{x}-2)(\sqrt{x}-3))/((x-9)(\sqrt{x}+3)).(x-9)/\sqrt{x}`
`=(x+5\sqrt{x}+6-x+5\sqrt{x}-6)/(\sqrt{x}(\sqrt{x}+3))`
`=(10\sqrt{x})/(\sqrt{x}(\sqrt{x}+3))`
`=10/(\sqrt{x}+3)`
