ALevel数学-序列Sequences

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　　Revision: Sequences

　　Introduction

　　In the sequence 2, 4, 6, 8, 10.., there is an obvious pattern. Such sequences can be expressed in terms of the nth term of the sequence. In this case, the nth term 2n. To find the 1st term, put n = 1 into the formula, to find the 4th term, replaces the n's by 4's: 4th term 2 × 4 = 8.

　　Example

　　What is the nth term of the sequence 2, 5, 10, 17, 26... ? The easiest way to find the nth term is by trial and error.

　　n | 1 - 2 - 3 - 4 - 5

　　n² | 1 - 4 - 9 - 16 - 25

　　n² + 1 | 2 - 5 - 10 - 17 - 26

　　This is the required sequence, so the nth term is n² + 1. For some sequences, there is no easy way of working out the nth term of a sequence, other than to try different possibilities.

　　Tips: if the sequence is going up in threes (e.g. 3, 6, 9, 12...), there will probably be a three in the formula, etc.

　　In many cases, square numbers will come up, so try squaring n, as above. Also, the triangular numbers formula often comes up.

　　Notation

　　The nth term of a sequence is usually written as Un. So in the last example, Un = n² + 1. The 5th term is therefore U5 = 25 + 1=26.

　　Recurrence relation

　　This is where the next term of a sequence is defined using the previous term(s). For example, the recurrence relation for 2, 4, 8, 16, 32, ... would be: U1 = 2, Un = 2(Un-1). This tells us that the first term, U1, is 2 and the next term of the sequence can be found by doubling the previous term.

　　Convergent Sequences

　　Sequences whose nth term approaches a finite number as n becomes larger are known as convergent sequences and the number to which the sequence converges are known as the limit of the sequence. For example: 10, 5, 2.5, 1.25, 0.625, ... converges towards the limit zero.

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