4.4.1 Language of Sequences & Series
Language of Sequences & Series
What is a progression/sequence?
- A progression (or sequence) is an ordered set of numbers with a rule for finding all the numbers in the progression
- The numbers in a sequence are called terms
- The terms of a progression are often referred to by letters with a subscript
What is a series?
- You get a series by summing up the terms in a progression
- We use the notation Sn to refer to the sum of the first n terms in the progression
ie. Sn = u1 + u2 + u3 + … + un
Increasing, decreasing and periodic progressions
- A progression is increasing if un+1 > un for all positive integers n – ie if every term is greater than the term before it
- A progression is decreasing if un+1 < un for all positive integers n – ie if every term is less than the term before it
- A progression is periodic if the terms repeat in a cycle
- The order (or period) of a periodic progression is the number of terms in each repeating cycle
Exam Tip
Look out for progressions defined by trigonometric functions – this can be a way of 'hiding' a periodic function. 
Worked example
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