s（n+1）-sn
=5/6(n+1)(n+4)-5/6n(n+3)
=5/6(n²+5n+4-n²-4n-3)
=5/6(n+1)
=5/6n+5/6
所以an是等差数列
已知数列an的前n项和为sn=5/6n(n+3),1:求证an为等差数列 2：设bn=a3n+a
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