设原式=S3S=3+3×3²+5×3^3+┈+（2n-1）×3 ∧n2S=3S-S=-1-2×3-2×3²-┈-2×3 ∧n-1 +(2n-1）×3 ∧n=-1-2×(3+3²+┈+3 ∧n-1)+(2n-1）×3 ∧n=-1+3-3^n+(2n-1）×3 ∧n=2+(2n-2)×3^nS=(n-1)×3^n+...谢了如果满意请给采纳谢谢