`\text{Chủ tus sửa lại đề : }`
`a ) 1/2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100`
`= 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100`
`= 1 - ( 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/99 - 100)`
`= 1 - 1/100`
`= 100/100 - 1/100`
`= 99/100`
`b ) 1/2.4+1/4.6+...+1/98.100`
`= 1/2 . ( 2/2.4 + 2/4.6 + ... + 2/98.100 )`
`= 1/2 . ( 1/2 - 1/4 + 1/4 - 1/6 + ... + 1/98 - 1/100 )`
`= 1/2 . ( 1/2 - 1/100 )`
`= 1/2 . ( 50/100 - 1/100 )`
`= 1/2 . 49/100`
`=49/200`