`a)` `4sqrt3+sqrt27-sqrt45+sqrt5`
`=4sqrt3+3sqrt3-3sqrt5+sqrt5`
`=7sqrt3-2sqrt5`
`b)` `sqrt{2+sqrt{9+4sqrt2}}`
`=sqrt{2+sqrt{8+4sqrt2+1}}`
`=sqrt{2+sqrt{(2sqrt2)^2+2·2sqrt2·1+1^2}}`
`=sqrt{2+sqrt{(2sqrt2 +1)^2}}`
`=sqrt{2+|2sqrt2+1|}`
`=sqrt{2+(2sqrt2+1)}` `(Vì ` `2sqrt2+1>0` `)`
`=sqrt{2+2sqrt2+1}`
`=sqrt{3+4sqrt2}`
`=sqrt{2+4sqrt2+1}`
`=sqrt{(sqrt2+1)^2}`
`=|sqrt2+1|`
`=sqrt2+1` `(Vì ` `sqrt2+1>0` `)`