Đáp án:
`\frac{1}{12} + \frac{1}{20} + \frac{1}{30} + ... + \frac{1}{9702}`
`= \frac{1}{3.4} + \frac{1}{4.5} + \frac{1}{5.6} + ... + \frac{1}{98.99}`
` =\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{98}- \frac{1}{99}`
`= \frac{1}{3} - \frac{1}{99}`
`= \frac{32}{99}`
Chúc học tốt!!!