原式=1/4(1×2×3×4-0×1×2×3)+1/4﹙2×3×4×5-1×2×3×4﹚+.1/4﹙n×﹙n+1﹚×﹙n+2﹚×﹙n+3﹚×﹙n+4﹚-﹙n-1﹚×n×﹙n+1﹚×﹙n+2﹚=1/4×n×﹙n+1﹚×﹙n+2﹚×﹙n+3﹚
1x2x3+2x3x4+.+nx(n+1)(n+2)=?
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