P
RE
C
ALCULUS
11
S
ECTION
1.3
G
EOMETRIC
S
EQUENCES
i) Terms, common ratio, number of terms in a
geometric sequence
ii) Geometric means
iii) Solving Algebraic sequences
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I) W
HAT
IS
A
G
EOMETRIC
S
EQUENCE
?
A sequence where each term after the first is
multiplied
by a common ratio
“r”
Not the same as an arithmetic sequence (Add)
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II) F
ORMULA
FOR
G
EOMETRIC
S
EQUENCE
:
The value of the “
n
th
” term
Value of the 1
st
term
Common Ratio
The rank of the term
- Divide any term by its previous term
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E
XAMPLE
1:
I
N
THE
GEOMETRIC
SEQUENCE
5, 10,
20, 40, …,
DETERMINE
a)
The common ratio (
r
)
b)
The 7
th
term (
t
7
)
c)
The general term (
t
n
)
a)
b)
c)
t
6
= 160
t
7
= 320
t
5
= 80
t
4
= 40
a
= 5
t
2
= 10
t
3
= 20
t
1
=
a
t
2
=
ar
t
3
=
ar·r = ar
2
t
4
=
ar·r·r = ar
3
t
5
=
ar·r·r·r = ar
4
t
6
=
ar·r·r·r·r = ar
5
t
7
=
ar·r·r·r·r·r = ar
6
E
X
: G
IVEN
THE
FOLLOWING
SEQUENCE
,
FIND
THE
20
TH
TERM
:
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E
X
:
GIVEN
THE
THREE
TERMS
IN
A
GEOMETRIC
SEQUENCE
,
FIND
THE
COMMON
RATIO
:
To find the common ratio, you can just take
any term and divide it by the previous term
So, the common ratio will be:
E
XAMPLE
2:
C
ONSIDER
THE
GEOMETRIC
SEQUENCE
4, -12, 36, -108, …
a)
Determine the 19
th
term.
forms a geometric sequence.
The
new
sequence
formed
is
1
,
2
,
4
which
is
a
geometric
sequence.
If
Solve for
x
.
E
X
:
THE
THIRD
TERM
OF
A
GEOMETRIC
SEQUENCE
IS
27
AND
THE
SIXTH
TERM
IS
64. F
IND
THE
COMMON
RATIO
:
Each term in the sequence is equal to the
previous term multiplied by the common ratio:
The sixth term is equal to both 64 and 27r
3
To find the terms in between, just multiply
27 by the common ratio.
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E
X
: G
IVEN
AND
FIND
THE
COMMON
RATIO
AND
THE
FIRST
TERM
:
Now solve for
the first term
E
XAMPLE
4:
F
IND
THE
COMMON
RATIO
FOR
A
GEOMETRIC
SEQUENCE
WITH
A
FIRST
TERM
OF
3/4
AND
A
THIRD
TERM
OF
27/16.
T
h
e
r
e
f
o
r
e
t
h
e
s
e
q
u
e
n
c
e
s
a
r
e
o
r
E
XAMPLE
6:
T
HE
FIRST
AND
SECOND
TERMS
OF
A
GEOMETRIC
SEQUENCE
HAVE
A
SUM
OF
15,
WHILE
THE
SECOND
AND
THIRD
TERMS
HAVE
A
SUM
OF
60. U
SE
AN
ALGEBRAIC
METHOD
TO
FIND
ALL
THREE
TERMS
.
Let the first three terms be
We will find
r
from the following system of two equations
in
two unknowns.
Dividing equation (2) by equation (1)
we can eliminate a.
H
OMEWORK
:
Assignment 2.2