1 /6×11+1/11×16+1/16×21+1/21×26+1/26×31+1/31×36 =

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1 /6×11+1/11×16+1/16×21+1/21×26+1/26×31+1/31×36 =
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1 /6×11+1/11×16+1/16×21+1/21×26+1/26×31+1/31×36 =
1 /6×11+1/11×16+1/16×21+1/21×26+1/26×31+1/31×36 =

1 /6×11+1/11×16+1/16×21+1/21×26+1/26×31+1/31×36 =
1/n(n+5)=(1/5)[1/n-1/(n+5)]
于是1/6x11=(1/5)x(1/6-1/11)……1/31x36=(1/5)x(1/31-1/36)
于是1 /6×11+1/11×16+1/16×21+1/21×26+1/26×31+1/31×36
=(1/5)(1/6-1/11+1/11-1/16+1/16-1/21+1/21-1/26+1/26-1/31+1/31-1/36)
=(1/5)(1/6-1/36)
=1/36

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