`a)\sqrt{(a-4)^2}`

`=|a-4|`

`=a-4` (vì `a>=4->a-4>=0)`

`b)\sqrt{(5-a)^2}+4a`

`=|5-a|+4a`

`=5-a+4a` (vì `a<5->5-a>0)`

`=5+3a`

`c)\sqrt{(a-1)^2}`

`=|a-1|`

`=a-1` (vì `a>=1->a-1>=0)`

`d)\sqrt{(2-a)^2}+a`

`=|2-a|+a` 

`=2-a+a` (vì `a<2->2-a>0)`

`=2`

`e)\sqrt{a^2+6a+9}`

`=\sqrt{(a+3)^2}`

`=|a+3|`

`=a+3` (vì `a>=-3->a+3>=0)`

`f)\sqrt{4a^2-4a+1}+2a`

`=\sqrt{(2a-1)^2}+2a`

`=|2a-1|+2a`

`=-(2a-1)+2a` (vì `a<1/2->2a-1<0)` 

`=1-2a+2a` 

`=1`

`g)\sqrt{a^2+2a+1}`

`=\sqrt{(a+1)^2}`

`=|a+1|`

`=a+1` (vì `a>=-1->a+1>=0)`

`h)\sqrt{9a^2-6a+1}+3a`

`=\sqrt{(3a-1)^2}+3a`

`=|3a-1|+3a`

`=-(3a-1)+3a` (vì `a<1/3->3a-1<0)`

`=1-3a+3a`

`=1`