Đáp án:
`a)sqrt{sqrt5-sqrt{9-sqrt{29+12sqrt5}}}`
`=sqrt{sqrt5-sqrt{9-sqrt{20+2.3.2sqrt5+9}}}`
`=sqrt{sqrt5-sqrt{9-sqrt{(2sqrt5+3)^2}}}`
`=sqrt{sqrt5-sqrt{9-3-2sqrt5}}`
`=sqrt{sqrt5-sqrt{6-2sqrt5}}`
`=sqrt{sqrt5-sqrt{5-2sqrt5+1}}`
`=sqrt{sqrt5-sqrt{(sqrt5-1)^2}}`
`=sqrt{sqrt5-sqrt5+1}`
`=sqrt1=1`
`b)5sqrt{25a^2}-25a(a<0)`
`=5|5a|-25a`
`=5.(-5a)-25a(a<0)`
`=-50a`