等差数列：an=a1+(n-1)dSn=n(a1+an)/2等比数列：an=a1q^(n-1)Sn=a1(1-q^n)/(1-q)
等差：an=a1+(n-1)d 求和：Sn=a1*n+n(n-1)d)/2等比：an=a1 x q^(n-1) 求和：Sn=a1(1-q^n)/(1-q)
等差数列an=a1+(n-1)dSn=n(a1+an)/2=na1+n(n-1)d/2
