Đáp án:
Giải thích các bước giải:
`S=1/3+1/9+1/18+1/39+1/45+1/63+...+1/14850`
`S=1/1.3+1/3.3+1/3.6+1/3.15+1/3.21+...+1/3.4950`
`S/3=1+1/3+1/6+1/15+1/21+...+1/4950`
`S/6=1/2+1/6+1/12+1/30+1/42+...+1/9900`
`S/6=(1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+...+1/99.100)-1/4.5`
`S/6=(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+...+1/99-1/100)-1/4-1/5`
`S/6=(1-1/100)-1/4-1/5` `S/6=99/100-(1/4+1/5)`
`S/6=99/100-9/20`
`S/6=27/50`
`S=27/50*6`
`S=81/25`
