`a) \sqrt{4( 3 - x )^2} - 20 =0`

`2\sqrt{( 3 - x )^2} = 20`

`\sqrt{( 3 - x )^2} = 10`

`| 3 - x | = 10`

`3 - x = 10  hoặc   3 - x = -10`

`x = -7  hoặc   x = 13`

`Vậy  pt  có  S = { -7 ; 13}`

`b) \sqrt{x^2 - 6x + 9} = \sqrt{5 - 2\sqrt{6}} + \sqrt{5 + 2\sqrt{6}}`

`\sqrt{( x - 3 )^2} = \sqrt{3 - 2\sqrt{3. 2} + 2 } + \sqrt{3 + 2\sqrt{3. 2} + 2}`

`| x - 3 | = \sqrt{(\sqrt{3}-\sqrt{2})^2} + \sqrt{(\sqrt{3}+\sqrt{2})^2}`

`| x - 3 | = \sqrt{3} - \sqrt{2} + \sqrt{3} + \sqrt{2}`

`| x - 3 | = 2\sqrt{3}`

`x - 3 = 2\sqrt{3}  hoặc   x - 3 = -2\sqrt{3}`

`x = 3 + 2\sqrt{3}  hoặc   x = 3 - 2\sqrt{3}`

`Vậy  pt  có  S = { 3 +- 2\sqrt{3} }`