Đáp án:+Giải thích các bước giải:
a, `C=1/(\sqrt3+\sqrt2-\sqrt6)-1/(\sqrt3+\sqrt2+\sqrt6)`
`=(\sqrt3+\sqrt2+\sqrt6)/((\sqrt3+\sqrt2+\sqrt6)(\sqrt3+\sqrt2-\sqrt6))-(\sqrt3+\sqrt2-\sqrt6)/((\sqrt3+\sqrt2+\sqrt6)(\sqrt3+\sqrt2-\sqrt6))`
`=(\sqrt3+\sqrt2+\sqrt6-(\sqrt3+\sqrt2-\sqrt6))/((\sqrt3+\sqrt2+\sqrt6)(\sqrt3+\sqrt2-\sqrt6))`
`=(\sqrt3+\sqrt2+\sqrt6-\sqrt3-\sqrt2+\sqrt6)/((\sqrt3+\sqrt2)^2-(\sqrt6)^2)`
`=(2\sqrt6)/(5+2\sqrt6-6)`
`=(2\sqrt6)/(2\sqrt6-1)`
`=(2\sqrt6(2\sqrt6+1))/((2\sqrt6-1)(2\sqrt6+1))`
`=(24+2\sqrt6)/23`
b, `D=((\sqrt6-\sqrt2)/(1-\sqrt3)-5/\sqrt5):1/(\sqrt5-\sqrt2)`
`=( (-\sqrt2(1-\sqrt3))/(1-\sqrt3)-\sqrt5).(\sqrt5-\sqrt2)`
`=(-\sqrt2-\sqrt5).(\sqrt5-\sqrt2)`
`=-(\sqrt2+\sqrt5).(\sqrt5-\sqrt2)`
`=-[(\sqrt5)^2-(\sqrt2)^2]=-3`
