Đáp án:
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`a,`
`D = 5^2/(1×6) + 5^2/(6×11) + ... + 5^2/(26 × 31)`
`-> D= 5 × [5/(1×6) + 5/(6×11) + ... + 5/(26×31)]`
`-> D = 5 × [1 - 1/6 + 1/6 - 1/11 + ... + 1/26 - 1/31]`
`-> D= 5 × [1 + (-1/6 + 1/6) + ... + (-1/26 + 1/26) - 1/31]`
`-> D =5 × [1 - 1/31]`
`-> D = 5 × 30/31`
`->D=150/31`
Vậy ...
`b,`
`K = 4/(11×16) + 4/(16×21) + ... + 4/(61×66)`
`-> K =4/5 × [1/11 - 1/16 + 1/16 - 1/21 + ... + 1/61 - 1/66]`
`-> K = 4/5 × [1/11 + (-1/16 + 1/16) + ... + (-1/61 + 1/61) - 1/66]`
`-> K = 4/5 × [1/11 - 1/66]`
`-> K = 4/5 × 5/66`
`->K=2/33`
Vậy ...
