`1/(1.3) + 1/(3.5)+1/(5.7) +....+1/(19.21)`
`= 1/2 . (2/(1.3) + 2/(3.5)+2/(5.7) +....+2/(19.21))`
`= 1/2 . (1-1/3+1/3-1/5+1/5-1/7+....+1/19 - 1/21)`
`= 1/2 . (1 - 1/21)`
`= 1/2 . 20/21`
`= 10/21`
`1/99-1/(99.97)-1/(97.95)-1/(95.93)-....-1/(5.3)-1/(3.1)`
`= 1/99 - (1/(99.97)+1/(97.95)+1/(95.93)+....+1/(5.3)+1/(3.1))`
`= 1/99 - (1/(1.3)+1/(3.5)+1/(5.7)+....+1/(95.97)+1/(97.99))`
`= 1/99 - (1/(1.3)+1/(3.5)+1/(5.7)+....+1/(95.97)+1/(97.99))`
`= 1/99 - (1/2 . (1/(1.3)+1/(3.5)+1/(5.7)+....+1/(95.97)+1/(97.99)))`
`= 1/99 - (1/2 . (1-1/3+1/3-1/5+1/5-1/7+....+1/95-1/97+1/97-1/99))`
`= 1/99 - (1/2 . (1-1/99))`
`= 1/99 - (1/2 . 98/99)`
`= 1/99 - 49/99`
`= -16/33`
(Chúc bạn học tốt)