`e) 3/35-(3/5-x)=2/7`
`3/5-x=3/35-2/7`
`3/5-x=3/35-10/35`
`3/5-x=-1/5`
`x=3/5--1/5`
`x=3/5+1/5`
`x=4/5`
`4)`
`a)` Ta có :
`1/(n(n+1))=((n+1)-n)/(n(n+1))=(n+1)/(n(n+1))-n/(n(n+1))=1/n-1/(n+1)`
`b)1/(1.2)+1/(2.3)+1/(3.4)+.....+1/(98.99)+1/(99.100)`
`=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+.....+1/98-1/99+1/99-1/100`
`=1-1/100`
`=99/100`
`5)`
`1/3-1/12-1/20-1/30-1/42-1/56-1/72-1/90-1/110=x-5/13`
`1/3-(1/(3.4)+1/(4.5)+1/(5.6)+1/(6.7)+.....+1/(9.10)+1/(10.11))=x-5/13`
`1/3-(1/3-1/4+1/4-1/5+1/5-1/6+....+1/9-1/10+1/10-1/11)=x-5/13`
`1/3-(1/3-1/11)=x-5/13`
`1/3-1/3+1/11=x-5/13`
`1/11=x-5/13`
`x=13/143+55/143`
`x=68/143`
