Đáp án:
`(1/(2-sqrt5)+2/(sqrt5+sqrt3)):1/sqrt{21-12sqrt3}`
`=((2+sqrt5)/(4-5)+(2(sqrt5-sqrt3))/(5-3)):1/sqrt{12-2.2sqrt{3}.3+9}`
`=(-(2+sqrt5)+2(sqrt5-sqrt3)):1/sqrt{(2sqrt3-3)^2}`
`=(-2-sqrt5+2sqrt5-2sqrt3):1/(2sqrt3-3)`
`=(sqrt5-2sqrt3-2)(2sqrt3-3)`
`=2sqrt{15}-3sqrt5-12+6sqrt3-4sqrt3+6`
`=2sqrt{15}-3sqrt5+2sqrt3-6`