設(shè)數(shù)列的首項為a(1),公差為d,且d＞0,則依題意,有：
［a(1)＋d］＋［a(1)＋6d］＝9［a(1)＋3d］＋［a(1)＋4d］＝20,
∴2a(1)＋7d＝10a(1)＋31d＝20.
由2a(1)＋7d＝10a(1)＋31d,得：8a(1)＋24d＝0,∴a(1)＝－3d.
將a(1)＝－3d代入到10a(1)＋31d＝20中,得：－30d＋31d＝20,∴d＝20,∴a(1)＝－60.
∴a(n)＝a(1)＋（n－1）d＝－60＋（n－1）×20＝20n－80.
即：a(n)＝20n－80.