Đáp án:
`H=5/14`.
Giải thích các bước giải:
`H=10/56+10/140+10/260+...+10/1400`
`=>H=5/28+5/70+5/130+...+5/700`
`=>H=5/4.7+5/7.10+5/10.13+...+5/25.28`
`=>H/5=1/4.7+1/7.10+1/10.13+...+1/25.28`
`=>(3H)/5=3/4.7+3/7.10+3/10.13+...+3/25.28`
`=>(3H)/5=1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28`
`=>(3H)/5=1/4-1/28`
`=>(3H)/5=3/14`
`=>3H=15/14`
`=>H=5/14`
Vậy `H=5/14`.
