4.
`A= 5/1xx6 + 5/6xx11 + 5/11xx16+...+ 5/36xx41`
`A= 1 -1/6 + 1/6 -1/11 + 1/11 -1/16+...+1/36 -1/41`
`A= 1 -1/41`
`A= 40/41`
5.
`B= 1/1.6 + 1/6.11 + 1/11.16+...+1/36.41`
`5B = 5(1/1.6+1/6.11 +1/11.16+...+1/36.41)`
`5B= 5/1.6 + 5/6.11 + 5/11.16 +...+ 5/36.41`
`5B= 1 -1/6 + 1/6-1/11 +1/11 -1/16+...+1/36-1/41`
`5B= 1 -1/41`
`5B= 40/41`
`B= 40/41 : 5`
`B= 40.41 . 1/5`
`B= 8/41`
6.
`C= 2/1.6 + 2/6.11 + 2/11.16+...+2/36.41`
`C = 2(1/1.6 + 1/6.11 + 1/11.16+...+1/36.41)`
`5C = 5.2 (1/1.6 + 1/6.11 +1/11.16+...+1/36.41)`
`5C= 2(5/1.6 + 5/6.11 + 5/11.16+...+5/36.41)`
`5C= 2.(1-1/6+1/6-1/11+1/11-1/16+...+1/36-1/41)`
`5C= 2.(1-1/41)`
`5C= 2. 40/41`
`5C= 80/41`
`C= 80/41 . 1/5`
`C= 16/41`
Vậy `C= 16/41`