2A = 2/(1.6) + 2/(3.10) + 2/(5.14) + ... + 2/(41.86)
2A = 1/(1.3) + 1/(3.5) + 1/(5.7) + ... + 1/(41.43)
4A = 2/(1.3) + 2/(3.5) + 2/(5.7) + ... + 2/(41.43)
4A = 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ... + 1/41 - 1/43
4A = 1/1 - 1/43
4A = 42/43
Vậy A = 21/86.