an＝n(n+1)＝n²+n,
∴Sn＝(1²+2²+3²+…+n²)+(1+2+3+…+n)
＝n(n+1)(2n+1)/6+n(n+1)/2
＝n(n+1)(n+2)/3.
∴1×1+2×3+3×4+…+98×99
＝98×99×100÷3
＝485100.