Question

What is Sn-1 in AP ?Explain.

Answer 1

sn is the sum of terms. $\\ sn=n/2[2f+(n-1)d]$

Answer 2

$S_n = the\ sum\ of\ first\ n\ terms\ of\ the\ series.\\\\Series:\ a,\ a+d,\ a+2d,\ a+3d,...\\\\T_n=n^{th}\ term=a+(n-1)d\\\\S_n=a+a+a+...n\ times\ + d*[1+2+3+....(n-1)\ terms]\\S_n=n*a+d*n(n-1)/2=[2a+(n-1)d]*n/2\\\\S_{n-1}=sum\ of\ the\ first\ (n-1)\ terms\\\\S_n-1=[2a+(n-2)d](n-1)/2$