Calculus 8.1 - University of Houston
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Transcript Calculus 8.1 - University of Houston
8.1: Sequences
Craters of the Moon National Park, Idaho
Photo by Vickie Kelly, 2008
Greg Kelly, Hanford High School, Richland, Washington
A sequence is a list of numbers written in an explicit order.
an a1, a2 , a3, ... , an , ...
nth term
Any real-valued function with domain a subset of the
positive integers is a sequence.
If the domain is finite, then the sequence is a finite sequence.
In calculus, we will mostly be concerned with infinite sequences.
A sequence is defined explicitly if there is a formula that
allows you to find individual terms independently.
an
Example:
1
n
n2 1
To find the 100th term, plug 100 in for n:
1
100
a100
1
1002 1 10001
A sequence is defined recursively if there is a formula that
relates an to previous terms.
Example:
b1 4
bn bn1 2 for all n 2
We find each term by looking at the term or terms before it:
b1 4
b2 b1 2 6
b3 b2 2 8
b4 b3 2 10
You have to keep going this
way until you get the term you
need.
An arithmetic sequence has a common difference
between terms.
Example: 5, 2, 1, 4, 7, ...
ln 2, ln 6, ln18, ln 54, ...
Arithmetic sequences can
be defined recursively:
or explicitly:
d 3
6
d ln 6 ln 2 ln
ln 3
2
an an1 d
an a1 d n 1
An geometric sequence has a common ratio between
terms.
Example: 1, 2, 4, 8, 16, ...
102 , 101 , 1, 10, ...
Geometric sequences can
be defined recursively:
or explicitly:
r 2
101
r 2
10
10
an an1 d
an a1 d n1
If the second term of a geometric sequence
is 6 and the fifth term is -48, find an explicit
rule for the nth term.
Example:
a1 r
48
a1 r
6
4
r 3 8
r 2
a2 a1 r 21
6 a1 2
3 a1
an 3 2
n 1
Sequence Graphing on the Ti-89
Change the graphing mode to “sequence”:
MODE
Graph…….
4
ENTER
n 1
an 1
n
n
Example: Plot
Y=
Use the alpha key to
enter the letter n.
Leave ui1 blank for
explicitly defined
functions.
WINDOW
WINDOW
GRAPH
The previous example was explicitly defined.
Now we will use a recursive definition to plot
the Fibonacci sequence.
a1 1
Y=
a2 1
an an2 an1
Use the alpha key to
enter the letters u
and n.
Enter the initial values
separated by a
comma (even though
the comma doesn’t
show on the screen!)
Enter the initial values
separated by a
comma (even though
the comma doesn’t
show on the screen!)
WINDOW
WINDOW
GRAPH
You can use F3 Trace
to investigate values.
We can also look at the
results in a table.
TBLSET
TABLE
Scroll down to see
more values.
TABLE
Scroll down to see
more values.
You can determine if a sequence converges by finding the
limit as n approaches infinity.
2n 1
Does an
converge?
n
2n 1
lim
n
n
2n 1
lim
n
n n
2n
1
lim
lim
n n
n n
20
2
The sequence converges and its
limit is 2.
Absolute Value Theorem for Sequences
If the absolute values of the terms of a sequence converge
to zero, then the sequence converges to zero.
Don’t forget to change back to function mode
when you are done plotting sequences.
p