Đáp án+Giải thích các bước giải:
`a)2/1.3+2/3.5+2/5.7+...+2/99.101`
`=1-1/3+1/3-1/5+1/5-1/7+...+1/99-1/101`
`=1+(-1/3+1/3)+(-1/5+1/5)+...+(-1/99+1/99)-1/101`
`=1+0+0+...+0+(-1/101)`
`=1-1/101`
`=(101-1)/101`
`=100/101`
`b)5/1.3+5/3.5+5/5.7+...+5/99.101`
`=5/(2).(2/1.3++2/3.5+2/5.7+...+2/99.101)`
`=5/(2)(1-1/3+1/3-1/5+1/5-1/7+...+1/99-1/101)`
`=5/2(1+(-1/3+1/3)+(-1/5+1/5)+...+(-1/99+1/99)-1/101)`
`=5/2(1+0+0+...+0+(-1/101))`
`=5/2(1-1/101)`
`=5/2((101-1)/101)`
`=5/2(100/101)`
`=250/101`
