`#khanhly2949`

`a,`

Có `:` `MN` $\parallel$ `EF` `(g``t)`

`=>(DM)/(DE)=(MN)/(EF)`

`=>(9,5)/(9,5+28)=8/x`

`=>9,5x=8 . 37,5`

`=>x=600/19`

`b,`

Có `:`

`B'A'` $\bot$ `A``A'`

`AB` $\bot$ `A``A'`

`=>B'A'` $\parallel$ `AB`

`=>(B'A')/(AB)=(A'O)/(OA)`

`=>(4,2)/x=3/6`

`=>3x=4,2 . 6`

`=>x=8,4`

Xét `\triangle OAB` vuông tại `O` có `:`

`OB^2=OA^2+AB^2`

`=>OB^2=6^2+(8,4)^2`

`=>0B^2=106,56`

`=>OB=(6\sqrt{74})/5`

`=>y=(6\sqrt{74})/5`