tính nhanh 9+99+999+9999+99999

  `9 + 99 + 999 + 9999 + 99999`

`= 10 - 1 + 100 - 1 + 1 000 - 1 + 10 000 - 1 + 100 000 - 1`

`= ( 10 + 100 + 1 000 + 10 000 + 100 000 ) - ( 1 + 1 + 1 + 1 + 1 )`

`= 111 110 - 5`

`= 111 105`

`#klyy~`