Đáp án :
`D=5/14`
Giải thích các bước giải :
`D=10/56+10/140+10/260+...+10/1400`
`<=>D=5/28+5/70+5/130+...+5/700`
`<=>D=5/(4*7)+5/(7*10)+5/(10*13)+...+5/(25*28)`
`<=>3/5D=3/(4*7)+3/(7*10)+3/(10*13)+...+3/(25*28)`
`<=>3/5D=1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28`
`<=>3/5D=1/4-1/28`
`<=>3/5D=7/28-1/28`
`<=>3/5D=3/14`
`<=>D=3/14:3/5`
`<=>D=3/14*5/3`
`<=>D=5/14`
Vậy : `D=5/14`