What is the sum of the arithmetic sequence?

Answer 1

The sum of an arithmetical sequence whose nth term is

U(n) = a + (n-1)*d is

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

or 1/2*n(a + l) where l is the last term in the sequence.

Answer 2

The sum of the first n terms of the sequence a + kd where a and d are constants and k = 1, 2, 3, ..., n isn*[a + d*(n+1)/2]