Đáp án:
`D = 1/100 - 1/(100 . 99) - 1/(99.98) - .... - 1/(3.2) = 1/(2.1) `
`D = 1/100 - (1/(100.99) + 1/(99.98) + 1/(98.97) +......+ 1/(3.2) +1/(2.1)) `
`D = 1/100 - (1/(2.1) + 1/(3.2) + ....... + 1/(99.98) + 1/(100.99))`
`D = 1/100 - ( 1 - 1/2 + 1/2 - 1/3 + 1/3 - ....... - 1/99 + 1/99 - 1/100)`
`D = 1/100 - ( 1- 1/100)`
`D = 1/100 - 99/100`
`D =-49/50`