Đáp án:
Giải thích các bước giải:
`a)`
`(sqrt5+2)((sqrt5+2)^2-8sqrt5)/(sqrt5-2)`
`= (sqrt5+2)(9+4sqrt5-8sqrt5)/(sqrt5-2)`
`= (sqrt5+2)(9-4sqrt5)/(sqrt5-2)`
`= ((9-4sqrt5)(sqrt5+2))/(sqrt5-2)`
`= (9sqrt5+9*2-4sqrt5sqrt5-4*2sqrt5)/(sqrt5-2)`
`= (sqrt5-2)/(sqrt5-2)`
`= 1`
`b)`
`((1-sqrt2)/(1+sqrt2) - (1+sqrt2)/(1-sqrt2)) \div sqrt{72}`
`= -((1-sqrt2)^2-(1+sqrt2)^2)/((1+sqrt2)(1-sqrt2)) \div 6sqrt2`
`= -(3+2sqrt2-3+2sqrt2)/(1^2-(sqrt2)^2)`
`= -(4sqrt2)/(-1)`
`= 4sqrt2 \div 6sqrt2`
`= 2/3`