We can add the terms of a sequence {an } and get an expression of the form: a1+ a2+ a3+ + an + which is called a series and denoted by However what does it mean by the sum of infinitely many terms? Example. We can try to add the terms of the series 1+2+3+ +n+ and get the cumulative sums 1, 3, 6, 10, , The nth sum n(n+1)/2 becomes very large as n increases