Đáp án:
`b) \ =20`
`d) \ =-5`
`f) \ =0`
Giải thích các bước giải:
`b)` `5/(sqrt5-2)-5/(sqrt5+2)`
`=(5(sqrt5+2)-5(sqrt5-2))/((sqrt5-2)(sqrt5+2))`
`=(5sqrt5+10-5sqrt5+10)/(5-4)`
`=20/1=20`
`d)` `((sqrt7-sqrt21)/(sqrt3-1)-(sqrt2-sqrt10)/(sqrt5-1)):1/(sqrt2+sqrt7)`
`=((-sqrt7(-1+sqrt3))/(sqrt3-1)-(-sqrt2(-1+sqrt5))/(sqrt5-1)):(sqrt2-sqrt7)/((sqrt2+sqrt7)(sqrt2-sqrt7))`
`=(-sqrt7+sqrt2):(sqrt2-sqrt7)/(-5)`
`=(sqrt2-sqrt7) . (-5)/(sqrt2-sqrt7)`
`=-5`
`f)` `3/(sqrt6-sqrt3)+4/(sqrt7+sqrt3)-1/(sqrt7-sqrt6)`
`=3/(sqrt3(sqrt2-1))+(4(sqrt7-sqrt3))/((sqrt7+sqrt3)(sqrt7-sqrt3))-(sqrt7+sqrt6)/((sqrt7-sqrt6)(sqrt7+sqrt6))`
`=(sqrt3)/(sqrt2-1)+(4(sqrt7-sqrt3))/4-(sqrt7+sqrt6)/1`
`=(sqrt3(sqrt2+1))/1+(sqrt7-sqrt3)-(sqrt7+sqrt6)`
`=sqrt6+sqrt3+sqrt7-sqrt3-sqrt7-sqrt6`
`=0`
