・ x^3 + x^2 + x
1+ 2
2+ 3+ 4+ 5
4+ 5+ 6+ 7+ 8+ 9
7+ 8+ 9+10+11+12+13+14
11+12+13+14+15+16+17+18+19+20
16+17+18+19+20+21+22+23+24+25+26+27
Σ{(x+1)^2+(x+1)-2}/2 - ∑{(x-1)^2+(x-1)}/2