`81.` `ĐK : x ge 0 `
`5/8.\sqrt{15x}-\sqrt{15x}+11=1/3.\sqrt{15x}`
`->5/8.\sqrt{15x}-\sqrt{15x}-1/3.\sqrt{15x}=-11`
`->\sqrt{15x}.(5/3-1-1/3)=-11`
`->\sqrt{15x}. 1/3=-11`
`->\sqrt{15x}=-33``(KTM)`
Do `xge0`
Vậy `x=∅`
`82.` `ĐK : x ge -5`
`\sqrt{4x+20}-\sqrt{x+5}-1/3\sqrt{9x+45}=4`
`->\sqrt{4(x+5)}-\sqrt{x+5}-1/3\sqrt{9(x+5)}=4`
`->2\sqrt{x+5}-\sqrt{x+5}-\sqrt{x+5}=4`
`->0\sqrt{x+5}=4`
Do `0\sqrt{x+5}=0`
Vậy `x=∅`
