Arithmetic and Geometric Progressions
Arithmetic and geometric progressions are particular types of sequences of numbers which occur
frequently in business calculations. This leaflet explains these terms and shows how the sums of
these sequences can be found.
An arithmetic progression is a sequence of numbers where each new term after the first is formed
by adding a fixed amount called the common difference to the previous term in the sequence.
For example the sequence
3, 5, 7, 9, 11 ...
is an arithmetic progression. Note that having chosen the first term to be 3, each new term is found
by adding 2 to the previous term, so the common difference is 2.
The common difference can be negative: for example the sequence
is an arithmetic progression with first term 2 and common difference −3.
In general we can write an arithmetic progression as follows:
a, a + d, a + 2d, a + 3d, ...
where the first term is a and the common difference is d. Some important results concerning
arithmetic progressions (a.p.) now follow:
The nth term of an a.p. is given by: a + (n − 1)d
The sum of the first n terms of an a.p. is
(2a + (n − 1)d)
The sum of the terms of an arithmetic progression is known as an arithmetic series
c© mathcentre May 19, 2003
A geometric progression is a sequence of numbers where each term after the first is found by
multiplying the previous term by a fixed number called the common ratio. The sequence
1, 3, 9, 27,...
is a geometric progression with first term 1 and common ratio 3. The common ratio could be a
fraction and it might be negative. For example, the geometric progression with first term 2 and
common ratio −1
In general we can write a geometric progression as follows:
a, ar, ar2, ar3, . . .
where the first term is a and the common ratio is r.
Some important results concerning geometric progressions (g.p.) now follow: