`#huy`
`a)1/(1.2)+1/(2.3)+1/(3.4)+...+1/(1999.2000)`
`=1.(1/1-1/2+1/2-1/3+1/3-1/4+...+1/1999-1/2000)`
`=1.(1/1-1/2000)`
`=1 . 1999/2000`
`=1999/2000`
`b)1/(1.4)+1/(4.7)+1/(7.10)+...+1/(100.103)`
`=1/3.(1/1-1/4+1/4-1/7+1/7-1/10+...+1/100-1/103)`
`=1/3.(1/1-1/103)`
`=1/3 . 102/103`
`=34/103`
`c)8/9-1/72-1/56-1/42-...-1/6-1/2`
`=8/9-(1/2+1/6+1/12+1/20+30+1/42+1/56+1/72)`
`=8/9-(1/(1.2)+1/(2.3)+1/(3.4)+1/(4.5)+1/(5.6)+1/(6.7)+1/(7.8)+1/(8.9)`
`=8/9-(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9)`
`=8/9-(1-1/9)`
`=8/9-8/9`
`=0`
