m,...........kj.j.j.
Đặt x=0,(9)=0,999..." role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">x=0,(9)=0,999...x=0,(9)=0,999... (Số thập phân vô hạn tuần hoàn với chu kì 9).Ta có:10x=9,9" role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">10x=9,910x=9,9Lấy 10x−x" role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">10x−x10x−x:10x−x=9,(9)−0,(9)" role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">10x−x=9,(9)−0,(9)10x−x=9,(9)−0,(9)Điều này có nghĩa là9x=9" role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">9x=99x=9hayx=1" role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">x=1x=1Vậy0,(9)=1" role="presentation" style="font-size: 12px; display: inline; font-style: normal; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; position: relative;">0,(9)=1