Tính bằng cách thuận tiện: A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/132 + 1/156.

Đáp án:

`A = \frac{1}{2} + \frac{1}{6} + \frac{1}{12} + \frac{1}{20} + ... + \frac{1}{132} + \frac{1}{156}`

`= \frac{1}{1.2} + \frac{1}{2.3} + \frac{1}{3.4} + \frac{1}{4.5} + ... + \frac{1}{11.12} + \frac{1}{12.13}`

`= 1 - \frac{1}{2} + \frac{1}{2} - \frac{1}{3} + \frac{1}{3} - \frac{1}{4} + \frac{1}{4} - \frac{1}{5} + ... + \frac{1}{11} - \frac{1}{12} + \frac{1}{12} - \frac{1}{13}`

`= 1 - \frac{1}{13}`

`= \frac{12}{13}`

Chúc học tốt!!!