For arithmetic sequence, we have two basic formulas



S = ( n / 2 ) ( A + T )



wherein

S = sum of n terms

n = number of terms

A = first term

T = last term



If the last term is not given instead only the first term is given we use the formula that visibleman_7887 has mentioned



S = ( n / 2 ) [ 2A + ( n - 1 ) * d ]



wherein

S = Sum of n terms

n = number of terms

A = first term

d = common difference

Source:
http://answers.yahoo.com/question/index?qid=20060830202603AASPvcs