What is the arithmetic sequence?

Answer 1

An arithmetic sequence depends on two numbers: the seed, a, and the common difference, d.
The first number in the sequence is the seed and each number is obtained from the previous one by adding the difference.

So U1 = a
U2 = a + d
U3 = U2 + d = a + d + d = a + 2d

U4 = U3 + d = a + 2d + d = a + 3d
and so on.

In general, Un = a + (n - 1)*d