Chapter MAA10
Sequences and Series
Algebra
Introduction to Sequences
A sequence is an ordered list of numbers that follow a specific pattern. Each number in a sequence is called a term. We typically denote the terms of a sequence using subscript notation:
A sequence can be defined by an explicit formula that gives the value of the
-th term directly, or by a recursive formula that defines each term based on previous terms.
Types of Sequences
Explicit Formula
Where
is a function that directly computes the
-th term.
Recursive Formula
Where
defines each term based on previous terms.
A series is the sum of the terms of a sequence. If
is a sequence, then the corresponding series is written as:
Arithmetic Sequences
An arithmetic sequence is a sequence where each term differs from the previous term by a constant value called the common difference (
).
In an arithmetic sequence:
•
is the first term
•
is the common difference:
• The sequence increases if
and decreases if
Problem: Find the 20th term of the arithmetic sequence: 5, 8, 11, 14, ...
1
First, identify the first term
and the common difference
:
2
Use the formula
to find the 20th term:
3
Therefore, the 20th term of the sequence is 62.
Arithmetic Series
An arithmetic series is the sum of the terms in an arithmetic sequence. For a finite arithmetic sequence with
terms, the sum
can be calculated using:
Sum of an Arithmetic Series
Where
is the first term,
is the nth term,
is the number of terms, and
is the common difference.
Derivation of the Sum Formula
1
Write out the sum in two different ways:
2
Add these two equations:
3
Simplify each pair of terms:
4
Solve for
:
5
Since
, we can rewrite the formula:
Problem: Find the sum of the first 30 positive integers.
1
This is an arithmetic sequence with
,
, and
.
2
The 30th term is
3
Using the formula
:
4
Therefore, the sum of the first 30 positive integers is 465.
Geometric Sequences
A geometric sequence is a sequence where each term is found by multiplying the previous term by a fixed non-zero number called the common ratio (
).
In a geometric sequence:
•
is the first term
•
is the common ratio:
• The sequence increases in magnitude if
and decreases if
• If
, all terms have the same sign as
• If
, the terms alternate in sign
Problem: Find the 8th term of the geometric sequence: 3, 6, 12, 24, ...
1
First, identify the first term
and the common ratio
:
2
Use the formula
to find the 8th term:
3
Therefore, the 8th term of the sequence is 384.
Geometric Series
A geometric series is the sum of the terms in a geometric sequence. For a finite geometric sequence with
terms and common ratio
, the sum
can be calculated using:
Sum of a Geometric Series
Where
The first formula is commonly used when
is the first term,
is the common ratio, and
is the number of terms.
The first formula is commonly used when
and the second when
.
Derivation of the Sum Formula
1
Write out the sum:
2
Multiply both sides by
:
3
Subtract the second equation from the first:
4
Factor out
on the left side:
5
Solve for
:
6
For
, we can rewrite this as:
Problem: Find the sum of the first 10 terms of the geometric series: 5, 15, 45, 135, ...
1
First, identify the first term
and the common ratio
:
2
Since
, use the formula
to find the sum:
3
Therefore, the sum of the first 10 terms is 147,620.
Infinite Geometric Series
When
, the sum of an infinite geometric series converges to a finite value:
This formula has many applications in calculus and real-world scenarios, such as calculating repeating decimals or determining the total distance traveled by a bouncing ball.
Problem: Express the repeating decimal
as a fraction.
1
Let
represent the sum of the infinite geometric series.
2
We can write this as:
3
The sum in parentheses is an infinite geometric series with
and
:
4
Therefore:
5
So
Applications of Sequences and Series
Sequences and series have numerous applications in mathematics and real-world scenarios.
Compound Interest
If
is the principal,
is the annual interest rate, and interest is compounded
times a year, the amount after
years is:
This is a geometric sequence with common ratio
.
Population Growth
A population that grows by a constant percentage follows a geometric sequence:
Where
is the initial population,
is the growth rate, and
is the number of time periods.
Summation of Natural Numbers
The sum of the first
natural numbers:
This is an arithmetic series with
and
.
Problem: A ball is dropped from a height of 10 meters. Each time it hits the ground, it rebounds to 60% of its previous height. Find the total distance the ball travels.
1
Let's break this down into up and down movements:
• Initial drop: 10 meters down
• First rebound:
meters up
• Second drop: 6 meters down
• Second rebound:
meters up
And so on...
2
The total distance is the sum of all these distances:
3
Grouping the up and down movements:
4
After the initial 10 meters, we have pairs of distances that form a geometric sequence:
5
The sum in parentheses is an infinite geometric series with
and
:
6
Therefore:
7
The ball travels a total distance of 40 meters.