Language of Sequences & Series (Cambridge O Level Additional Maths)

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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. 

u subscript 1 space equals space 5 space minus space 2 open parentheses 1 close parentheses space equals space 3
u subscript 2 space equals space 5 space minus space 2 open parentheses 2 close parentheses space equals space 1
u subscript 3 space equals space 5 space minus space 2 open parentheses 3 close parentheses space equals space minus 1
u subscript 4 space equals space 5 space minus space 2 open parentheses 4 close parentheses space equals space minus 3
u subscript 5 space equals space 5 space minus space 2 open parentheses 5 close parentheses space equals space minus 5

The first five terms are 3, 1, -1, -3, -5

To find Sadd the first five terms of the progression together. 

3 space plus space 1 space plus space open parentheses negative 1 close parentheses space plus space open parentheses negative 3 close parentheses space plus space open parentheses negative 5 close parentheses

S= -5

Sigma Notation

What is sigma notation?

  • Sigma notation is used to show the sum of a certain number of terms in a sequence
  • The symbol Σ is the capital Greek letter sigma
  • Σ stands for ‘sum’
    • The expression to the right of the Σ tells you what is being summed, and the limits above and below tell you which terms you are summing

Sigma notation explained

  • Be careful, the limits don’t have to start with 1
    • For example sum from k space equals space 0 to 4 of left parenthesis 2 k plus 1 right parenthesis  or  sum from k space equals space 7 to 14 of left parenthesis 2 k minus 13 right parenthesis
    • r and k are commonly used variables within sigma notation

Examiner Tip

  • Sigma notation will not be tested in the exam but understanding it will help you to further understand series 

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Amber

Author: Amber

Expertise: Maths

Amber gained a first class degree in Mathematics & Meteorology from the University of Reading before training to become a teacher. She is passionate about teaching, having spent 8 years teaching GCSE and A Level Mathematics both in the UK and internationally. Amber loves creating bright and informative resources to help students reach their potential.