`A = 21/(8xx13) − 30/(13xx17) + 12/(17xx29) + 64/(29xx35) − 8/(35xx43) − 99/(43xx56)`

`A = (1/8 + 1/13) − (1/13 + 1/17) + (1/17 − 1/29) + (1/29 + 1/35) − (1/35 − 1/43) − (1/43 + 1/56)`

`A = 1/8 + 1/13 − 1/13 - 1/17 + 1/17 − 1/29 + 1/29 + 1/35 − 1/35 + 1/43 − 1/43 - 1/56`

`A = 1/8 + (1/13 - 1/13) - (1/17 - 1/17) - (1/29 - 1/29) + (1/35 - 1/35) + (1/43 - 1/43) - 1/56`

`A = 1/8 - 1/56`

`A = 7/56 - 1/56`

`A = 6/56`

`A = 3/28`

Vậy `A = 3/28`