`1)((sqrt{14}-sqrt7)/(1-sqrt2)+(sqrt{15}-sqrt5)/(1-sqrt3)):1/(sqrt7-sqrt5)`
`=((sqrt7(sqrt2-1))/(1-sqrt2)+(sqrt5(sqrt3-1))/(1-sqrt3):1/(sqrt7-sqrt5)`
`=(-sqrt7-sqrt5):1/(sqrt7-sqrt5)`
`=-(sqrt7+sqrt5).(sqrt7-sqrt5)`
`=-(7-5)`
`=-2`
`2)B=(2sqrtx-9)/(x-5sqrtx+6)-(sqrtx+3)/(sqrtx-2)-(2sqrtx+1)/(3-sqrtx)`
`=(2sqrtx-9-x+9+2x-3sqrtx-2)/(x-5sqrtx+6)`
`=(x-sqrtx-2)/(x-5sqrtx+6)`
`=((sqrtx-2)(sqrtx+1))/((sqrtx-2)(sqrtx-3))`
`=(sqrtx+1)/(sqrtx-3)`
`x=11+6sqrt2`
`=(3+sqrt2)^2`
`=>B=(4+2sqrt2)/(sqrt2)`
`=2+2sqrt2`
`3)5x^4+4x^2-1=0`
Đặt `t=x^2(t>=0)`
`pt<=>5t^2+4t-1=0`
`a-b+c=0`
`=>t_1=-1(l),t_2=1/5(tm)`
`<=>x=+-sqrt{1/5}`
Vậy `S={-sqrt{1/5},+sqrt{1/5}}`
