S1=1+2S2=2+3+4S3=3+4+5+6S4=4+5+6+7+8.....Sn=n+(n+1)+(n+2)+...+2n等差数列，总共n+1项Sn=(n+1)*(n+2n)/2=3n(n+1)/2可以分别把n=1,2,3,4代入验算，可得结果正确
