A Sequence is a set of things (usually numbers) that are in order.
In an Arithmetic Sequence the difference between one term and the next is a constant.
Example: 1, 4, 7, 10, 13, 16, 19, 22, 25, ...
This sequence has a difference of 3 between each number.
Suppose in an arithmetic sequence:
x1 is the first term, and
d is the difference between the terms (called the "common difference")
Then the nth term in the arithmetic sequence is:
xn = x1 + d * (n-1)
The sum of x1 + x2 + x3 + ... + xn is:
Sn = ( x1 + xn ) * n / 2
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