tính C=1/1.2.3+1/2.3.4+1/3.4.5+...+1/98.99.100

`C = \frac{1}{1.2.3} + \frac{1}{2.3.4} + ... + \frac{1}{99.100.101}`

`C = \frac{1}{2} ( \frac{2}{1.2.3} + \frac{2}{2.3.4} + ... + \frac{2}{99.100.101} ) `

`C = \frac{1}{2} ( \frac{1}{1.2} - \frac{1}{2.3} + \frac{1}{2.3} - \frac{3.4} + ... + \frac{1}{99.100} - \frac{1}{100.101} )`

`C = \frac{1}{2} ( \frac{1}{2} - \frac{1}{10100} )`

`C = \frac{1}{2} . \frac{5049}{10100}`

`C = \frac{5049}{20200}`