Question

等差数列前n项和写成二次函数是怎么个个来着

Answer 1

等差数列：

Sn=na1+n(n-1)d/2=na1+（dn^2)/2-nd/2=(d/2)*n^2+(a1-d/2)n

A=d/2 B=a1-d/2

等比数列：

若q≠1

Sn=a1(1-q^n)/(1-q)=(a1/(q-1))*q^n-a1/(q-1)

A=a1/(q-1) B=-a1/(q-1)