The sum to n-terms of the series 2,5,10,17,26,… is
Sn=2+5+10+17+…+Tn−1+Tn-------1 differences of the terms 3,5,7,... are in A.P
Sn= 2+5+10+.......+Tn−1+Tn----21−2⇒0=2+3+5+7+…+(n−1)term]−TnTn=2+n-1223+n-22Sn=∑tn=∑n2+1=nn+12n+16+n=n2n2+3n+76