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Đáp án: `b) 8`

`c) -3`

Giải thích các bước giải:

`b) sqrt(49+20sqrt6)-sqrt(33-12sqrt6)`

`=sqrt(24+2.5.2sqrt6+25)-sqrt(24-2.3.2sqrt6+9)`

`=sqrt((2sqrt6)^2+2.5.2sqrt6+5^2)-sqrt((2sqrt6)^2-2.3.2sqrt6+3^2)`

`=sqrt(2sqrt6+5)^2-sqrt(2sqrt6-3)^2=2sqrt6+5-(2sqrt6-3)`

`=2sqrt6+5-2sqrt6+3=8`

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`c) (sqrt75-sqrt21)/(5-sqrt7)+(sqrt3)/(2+sqrt3)-(9)/(sqrt3)`

`=(5sqrt3-sqrt7.sqrt3)/(5-sqrt7)+(sqrt3.(2-sqrt3))/((2+sqrt3)(2-sqrt3))-3sqrt3`

`=((5-sqrt7).sqrt3)/(5-sqrt7)+(2sqrt3-3)/(4-3)-3sqrt3`

`=sqrt3+(2sqrt3-3)-3sqrt3=-3`