`a)``x^2=81`

`=>x^2=(+-9)^2`

`=>[(x=9),(x=-9):}`

Vậy `x in {9;-9}`

`b)``(x-1)^2=sqrt{(-9/16)^2}`

`=>(x-1)^2=|-9/16|`

`=>(x-1)^2=9/16`

`=>(x-1)^2=(+-3/4)^2`

`=>[(x-1=3/4),(x-1=-3/4):}`

`=>[(x=7/4),(x=1/4):}`

Vậy `x in {7/4;1/4}`

`c)``x-2sqrt{x}=0`

`=>sqrt{x}(sqrt{x}-2)=0`

`=>[(sqrt{x}=0),(sqrt{x}-2=0):}`

`=>[(x=0),(sqrt{x}=2):}`

`=>[(x=0),(x=4):}`

Vậy `x in {0;4}`

`d)``x=sqrt{x}`

`=>x-sqrt{x}=0`

`=>sqrt{x}(sqrt{x}-1)=0`

`=>[(sqrt{x}=0),(sqrt{x}-1=0):}`

`=>[(x=0),(sqrt{x}=1):}`

`=>[(x=0),(x=1):}`

Vậy `x in {0;1}`