Language of Sequences & Series
What is a progression?
- A progression (also called a sequence) is an ordered set of numbers with a rule for finding all of the numbers in the sequence
- For example 1, 3, 5, 7, 9, … is a sequence with the rule ‘start at one and add two to each number’
- The numbers in a progression are often called terms
- The terms of a progression are often referred to by letters with a subscript
- This will often be the letter u
- So in the progression above, u1 = 1, u2 = 3, u3 = 5 and so on
- Each term in a progression can be found by substituting the term number into formula for the nth term
What is a series?
- You get a series by summing up the terms in a progression
- E.g. For the sequence 1, 3, 5, 7, … the associated series is 1 + 3 + 5 + 7 + …
- We use the notation Sn to refer to the sum of the first n terms in the series
- Sn = u1 + u2 + u3 + … + un
- So for the series above S5 = 1 + 3 + 5 + 7 + 9 = 25
Worked example
Determine the first five terms and the value of S5 in the progression with terms defined by un = 5 - 2n.
Substitute n in for each term that you want to find.
The first five terms are 3, 1, -1, -3, -5
To find S5 add the first five terms of the progression together.
S5 = -5