2 câu trả lời
`1+ 1-2+5-4+6-7+9-1+3+9-1+3-1`
`= (1+1 - 1 -1 ) + ( 9 - 2- 7)+ (5 - 1 - 4) + ( 9 - 2- 7) + 9 + 3 + 6 + 3`
=` 0 + 0 + 0 + 12 + 9`
=` 21`
1+ 1-2+5-4+6-7+9-1+3+9-1+3-1
= (5 - 1 - 4) + ( 9 - 2- 7) + (1+1 - 1 -1 ) + 9 + 3 + 6 + 3
= 0 + 0 + 0 + 12 + 9
= 21
