`{(x+2y=1),(2x-y=-8):}`

`<=>{(x=1-2y),(2x-y=-8):}`

`<=>{(x=1-2y),(2(1-2y)-y=-8):}`

`<=>{(x=1-2y),(2-4y-y=-8):}`

`<=>{(x=1-2y),(-5y=-10):}`

`<=>{(x=1-2y),(y=2):}`

`<=>{(x=-3),(y=2):}`

Vậy phương trình có nghiệm duy nhất là:

`(x;y)=(-3;2)`

`(\sqrt{75}+2\sqrt{27}-3\sqrt{12})\sqrt{3}`

`=(\sqrt{25.3}+2\sqrt{9.3}-3\sqrt{4.3)\sqrt{3}`

`=(\sqrt{25}.\sqrt{3}+2.\sqrt{9}.\sqrt{3}-3.\sqrt{4}.\sqrt{3})\sqrt{3}`

`=(5\sqrt{3}+6\sqrt{3}-6\sqrt{3})\sqrt{3}`

`=5\sqrt{3}.\sqrt[3}`

`=15`

`0,6xx7+1,2xx45+1,8`

`=0,6xx7+0,6xx2xx45+0,6xx3`

`=0,6xx7+0,6xx90+0,6xx3`

`=0,6xx(7+90+3)`

`=0,6xx100`

`=60`