3+6+10+15+……+(1/2)(n+1)(n+2)=(1/2)〔2×3＋2×3＋2×3＋……＋(n+1)(n+2)〕=(1/6)[18+3×4×5－2×3×4＋4×5×6－3×4×5＋……＋(n+1)(n+2)(n+3)-n(n+1)(n+2)]= (1/6)*(n+1)(n+2)(n+3)-1