Đáp án`+`Giải thích các bước giải:
`1` $\sqrt{17 - 4\sqrt{15}}$ `-` $\sqrt{23 - 4\sqrt{15}}$
`=` $\sqrt{12 - 2.2.\sqrt{3}.\sqrt{5}+5}$ `-` $\sqrt{20 - 2.2.\sqrt{3}.\sqrt{5}+3}$
`=` $\sqrt{(2√3)^2 - 2.2.\sqrt{3}.\sqrt{5}+(√5)^2}$ `-` $\sqrt{(2√5)^2 - 2.2.\sqrt{3}.\sqrt{5}+(√3)^2}$
`=` $\sqrt{(2\sqrt{3}-\sqrt{5})^2}$ `-` $\sqrt{(2\sqrt{5}-\sqrt{3})^2}$
`=` `|2 `$\sqrt{3}$ `-` $\sqrt{5}$ `|` `-` `|2`$\sqrt{5}$ `-` $\sqrt{3}$ `|`
`=` `2`$\sqrt{3}$ `-` $\sqrt{5}$ `-` `2`$\sqrt{5}$ `+` $\sqrt{3}$
`=` `3`$\sqrt{3}$ `-` `3`$\sqrt{5}$
