The sum of these n numbers is sn = a+(a+b)+...+[a+(n-1) b]
=na+[b+2b+....+(n- 1)b]
=na+b[ 1+2+....+....+(n- 1)]
= na+b[(n- 1)+(n- 1)(n-2)/2]
=na+b(n- 1) n/2