Answer

`1.`

`a, \sqrt{36} - \sqrt{196} + \sqrt{81}`

`= \sqrt{6^{2}} - \sqrt{14^{2}} + \sqrt{9^{2}}`

`= |6| - |14| + |9|`

`= 6 - 14 + 9`

`= -8 + 9`

`= 1` 

`b, (\sqrt{5} - 2\sqrt{3} + \sqrt{45}) . \sqrt{5} + 60`

`= (\sqrt{5} - 2\sqrt{3} + 3\sqrt{5}) . \sqrt{5} + 60`

`= (4\sqrt{5} - 2\sqrt{3}) . \sqrt{5} + 60`

`= 4\sqrt{5} . \sqrt{5} - 2\sqrt{3} . \sqrt{5} + 60`

`= 20 - 2\sqrt{15} + 60`

`= 80 - 2\sqrt{15}`

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`2`

`\sqrt{3x} - 2\sqrt{12} = \sqrt{75}` `ĐK : x >= 0`

`=> \sqrt{3x} - 4\sqrt{3} = 5\sqrt{3}`

`=> \sqrt{3x} = 5\sqrt{3} + 4\sqrt{3}`

`=> \sqrt{3x} = 9\sqrt{3}`

`=> \sqrt{3x} = \sqrt{3^{5}}`

`=> 3x = 3^{5}`

`=> 3x = 243`

`=> x = 243 : 3`

`=> x = 81`

$\text{Vậy}$ `x = 81`

Bài 1:

a, √36 - √196 + √81     

= 6 - 14 + 9 =1

b, (√5 - 2√3 + √45) √5 + 60

= 5 - 2√15 + 15 + 60

= 80 - 2√15

Bài 2: Đk: x ≥ 0

√3x - 2√12 = √75

→ √3x - 4√3 = 5√3

→ √x - 4 = 5

→ √x = 9 

→ x = 81 ( tm)