Đáp án:+Giải thích các bước giải:
1) `1/(1-\sqrt2)+1/(1+\sqrt2)=(1+\sqrt2)/((1-\sqrt2)(1+\sqrt2))+(1-\sqrt2)/((1-\sqrt2)(1+\sqrt2)`
`=(1+\sqrt2)/(1-(\sqrt2)^2)+(1-\sqrt2)/(1-(\sqrt2)^2)`
`=-(1+\sqrt2)-(1-\sqrt2)`
`=-1-\sqrt2-1+\sqrt2=-2`
2) `1/(1-\sqrt5)+1/(1+\sqrt5)=(1+\sqrt5)/((1-\sqrt5)(1+\sqrt5))+(1-\sqrt5)/((1-\sqrt5)(1+\sqrt5))`
`= (1+\sqrt5+1-\sqrt5)/(1-(\sqrt5)^2`
`=2/(-4)=-1/2`
3) `4/(1-\sqrt3)+(\sqrt3-1)/(\sqrt3+1)`
`=(4(1+\sqrt3))/((1-\sqrt3)(1+\sqrt3))+(\sqrt3-1)^2/((\sqrt3+1)(\sqrt3-1)`
`=-(4+\4sqrt3)/(2)+(4-2\sqrt3)/2`
`=(-4-4\sqrt3+4-2\sqrt3)/2=-(6\sqrt3)/2=-3\sqrt3`
