Đáp án:
`-`
Giải thích các bước giải:
`1/6+1/66+1/176+...+1/((5n+1)(5n+6))`
`=1/1.6+1/6.11+1/11.16+...+1/((5n+1)(5n+6))`
`=1/5(5/1.6+5/6.11+5/11.16+...+5/((5n+1)(5n+6)))`
`=1/5(1-1/6+1/6-1/11+1/11-1/16+...+1/(5n+1)-1/(5n+6))`
`=1/5(1-1/(5n+6))`
`=1/5((5n+6)/(5n+6)-1/(5n+6))`
`=1/5*(5n+5)/(5n+6)`
`=(n+1)/(5n+6)`