Đáp án:
`8/9 - 1/72 - 1/56 - 1/42 -1/30 - 1/20 - 1/12 - 1/6 - 1/2`
`= 8/9 - 1/(9.8) - 1/(8.7) - 1/(7.6) - 1/(6.5) - 1/(5.4) - 1/(4.3) - 1/(3.2) - 1/(2.1)`
`= 8/9 - (1/(9.8) + 1/(8.7) + 1/(7.6) + 1/(6.5) + 1/(5.4) + 1/(4.3) + 1/(3.2) + 1/(2.1))`
`= 8/9 - ((8 - 9)/(9.8) + (8-7)/(8.7) + (7-6)/(7.6) + (6-5)/(6.5) + (5-4)/(5.4) + (4-3)/(4.3) + (3-2)/(3.2) + (2-1)/(2.1))`
`= 8/9 - (9/(9.8) - 8/(9.8) + 8/(8.7) - 7/(8.7) + 7/(7.6) - 6/(7.6) + 6/(6.5) - 5/(6.5) + 5/(5.4) - 4/(5.4) + 4/(4.3) - 3/(4.3) + 3/(3.2) - 2/(3.2) + 2/(2.1) - 1/(2.1))`
`= 8/9 - (1/8 - 1/9 + 1/7 - 1/8 + 1/6 - 1/7 + 1/5 - 1/6 + 1/4 - 1/5 + 1/3 - 1/4 + 1/2 - 1/3 + 1 - 1/2)`
`= 8/9 - (1 - 1/9)`
`= 8/9 - (9/9 - 1/9)`
`= 8/9 - 8/9`
`= 0`
