Đáp án:
`A=15/8`
Giải thích các bước giải:
`A=1+1/3+1/6+1/10+1/15+...+1/120`
`A=1+2/6+2/12+2/20+2/30+...+2/240`
`A=2/1.2+2/2.3+2/3.4+2/4.5+2/5.6+...+2/15.16`
`A/2=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+...+1/15.16`
`A/2=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+...+1/15-1/16`
`A/2=1-1/16`
`A/2=15/16`
`A=15/8`