sequences finding a rule
TRANSCRIPT
A sequence is a set of terms, in a definite order, where the terms are obtained by some rule.
A finite sequence ends after a certain number of terms.
An infinite sequence is one that continues indefinitely.
For example: 1, 3, 5, 7, …(This is a sequence of odd
numbers)1st term = 2 x 1 – 1 = 1
2nd term = 2 x 2 – 1 = 33rd term = 2 x 3 – 1 = 5
nth term = 2 x n – 1 = 2n - 1
. .
. .
. .
+ 2
+ 2
NOTATION1st term = u
2nd term =u3rd term =u
nth term =u
. .
. .
. .
1
2
3
n
OR1st term = u
2nd term =u3rd term =u
nth term =u
. .
. .
. .
0
1
2
n-1
FINDINGTHE FORMULA
FORTHE TERMS OFA SEQUENCE
A recurrence relation defines the first term(s) in the sequence and the relation between successive terms.
u = 5u = u +3 = 8u = u +3 = 11
u = u +3 = 3n + 2
.
.
.
1
2
3
n+1
For example:
5, 8, 11, 14, …
1
2
n
What to look forwhen looking for the rule
defining a sequence
Constant difference: coefficient of n is the difference2nd level difference: compare with square numbers(n = 1, 4, 9, 16, …)
3rd level difference: compare with cube numbers(n = 1, 8, 27, 64, …)
None of these helpful: look for powers of numbers(2 = 1, 2, 4, 8, …)Signs alternate: use (-1) and (-1)
-1 when k is odd +1 when k is even
kk
2
3
n - 1
EXAMPLE:
Find the next three terms in the sequence 5, 8, 11, 14, …
EXAMPLE:
The nth term of a sequence is given by x =
a) Find the first four terms of the sequence.
b) Which term in the sequence is ?
c) Express the sequence as a recurrence relation.
1__
2nn
1
1024____
EXAMPLE:
Find the nth term of the sequence +1, -4, +9, -16, +25, …
EXAMPLE:
A sequence is defined by a recurrence relation of the form:M = aM + b.Given that M = 10, M = 20, M = 24, find the value of a and
thevalue of b and hence find M .
n + 1
1 32
4