Algebra1 Geometric Sequences

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Algebra1 Geometric Sequences
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Warm Up
Write a function to describe each of the
following graphs.
1) The graph of f (x) x2 - 3 translated 7 units
up
2) The graph of f (x) 2x2 6 narrowed and
translated 2 units down
1) f (x) x2 4
2) f (x) 3x2 4
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Geometric Sequences
Bungee jumpers can use geometric sequences to
calculate how high they will bounce. The table
shows the heights of a bungee jumpers bounces.
The height of the bounces shown in the table
above form a geometric sequence. In a geometric
sequence , the ratio of successive terms is the
same number r, called the common ratio.
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Geometric Sequences
Find the next three terms in each geometric
sequence.
A) 1, 3, 9, 27,
Step1 Find the value of r by dividing each
term by the one before it.
3 3 9 3 27 3 1 3
9
The value of r is 3.
5
Step2 Multiply each term by 3 to find the next
three terms.
The next three terms are 81, 243, and 729.
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B) -16, 4, -1, 1 , 4
Step1 Find the value of r by dividing each
term by the one before it.
4 -1 -1 -1 1/4 -1 -16 4
4 4 -1 4
The value of r is -1. 4
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Step2 Multiply each term by -1 to find the
next 4
three terms.
The next three terms are -1, 1, and 1.
16 64 256
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Now you try!
Find the next three terms in each geometric
sequence.
1a) 5, -10, 20, -40, 1b) 512, 384, 288,

1a) 80, -160, 320 1b) 216, 162, 121.5
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Geometric sequences can be thought of as
functions. The term number, or position in the
sequence, is the input of the function, and the
term itself is the output of the function.
To find the output an of a geometric sequence
when n is a large number, you need an equation,
or function rule. Look for a pattern to find a
function rule for the sequence above.
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The pattern in the table shows that to get the
nth term, multiply the first term by the common
ratio raised to the power n - 1.
If the first term of a geometric sequence is a 1
, the nth term is a n , and the common ratio is
r, then
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Finding the nth Term of a Geometric Sequence
A) The first term of a geometric sequence is 128,
and the common ratio is 0.5. What is the 10th
term of the sequence?
an a1rn - 1
Write the formula.
a10 (128)(0.5)10 - 1
Substitute 128 for a1 , 10 for n, and 0.5 for r.
a10 (128)(0.5)9
Simplify the exponent.
a10 0.25
Use a calculator.
The 10th term of the sequence is 0.25.
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B) For a geometric sequence, a 1 8 and r 3.
Find the 5th term of this sequence.
an a1rn - 1
Write the formula.
a5 (8)(3)5 - 1
Substitute 8 for a1 , 5 for n, and 3 for r.
a5 (8)(3)4
Simplify the exponent.
a5 648
Use a calculator.
The 5th term of the sequence is 648.
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C) What is the 13th term of the geometric
sequence 8, -16, 32, -64, ?
Step1 Find the value of r by dividing each
term by the one before it.
-16 -2 32 -2 -64 -2 8
-16 32
The value of r is -2.
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Step2 Plug the value of r in the following
formula.
an a1rn - 1
Write the formula.
a13 (8)(-2)13 - 1
Substitute 8 for a1 , 13 for n, and -2 for r.
a13 (8)(-2)12
Simplify the exponent.
a13 32,768
Use a calculator.
The 13th term of the sequence is 32,768.
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Now you try!
2) What is the 8th term of the sequence 1000,
500, 250, 125, ?
2) 7.8125
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Sports Application
A bungee jumper jumps from a bridge. The diagram
shows the bungee jumpers height above the ground
at the top of each bounce. The heights form a
geometric sequence. What is the bungee jumpers
height at the top of the 5th bounce?
The value of r is 0.4.
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an a1rn - 1
Write the formula.
a5 (200)(0.4)5 - 1
Substitute 200 for a1 , 5 for n, and 0.4 for r.
a5 (200)(0.5)4
Simplify the exponent.
a5 5.12
Use a calculator.
The height of the 5th bounce is 5.12 feet.
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Now you try!
3) The table shows a cars value for 3 years
after it is purchased. The values form a
geometric sequence. How much will the car be
worth in the 10th year?
3) 1342.18
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Assessment
Find the next three terms in each geometric
sequence.
1) 2, 4, 8, 16,
2) 400, 200, 100, 50,
3) 4, -12, 36, -108,
4) -2, 10, -50, 250,

• 32, 64, 128
• 2) 25, 12.5, 6.25
• 3) 324, -972, 2916
• 4)-1250, 6250, -31,250

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5) The first term of a geometric sequence is 1,
and the common ratio is 10. What is the 10th term
of the sequence?
6) What is the 11th term of the geometric
sequence 3, 6, 12, 24, ?
5) 1,000,000,000 6) 3072
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7) In the NCAA mens basketball tournament, 64
teams compete in round 1. Fewer teams remain in
each following round, as shown in the graph,
until all but one team have been eliminated. The
numbers of teams in each round form a geometric
sequence. How many teams compete in round 5?
7) 4
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Find the missing term(s) in each geometric
sequence.
8) 20, 40,___,____ ,
9) ___, 6, 18,___,
8) 80, 160 9) 2, , , 54
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Lets review
Geometric Sequences
Bungee jumpers can use geometric sequences to
calculate how high they will bounce. The table
shows the heights of a bungee jumpers bounces.
The height of the bounces shown in the table
above form a geometric sequence. In a geometric
sequence , the ratio of successive terms is the
same number r, called the common ratio.
24
Geometric Sequences
Find the next three terms in each geometric
sequence.
A) 1, 3, 9, 27,
Step1 Find the value of r by dividing each
term by the one before it.
3 3 9 3 27 3 1 3
9
The value of r is 3.
25
Step2 Multiply each term by 3 to find the next
three terms.
The next three terms are 81, 243, and 729.
26
Geometric sequences can be thought of as
functions. The term number, or position in the
sequence, is the input of the function, and the
term itself is the output of the function.
To find the output an of a geometric sequence
when n is a large number, you need an equation,
or function rule. Look for a pattern to find a
function rule for the sequence above.
27
The pattern in the table shows that to get the
nth term, multiply the first term by the common
ratio raised to the power n - 1.
If the first term of a geometric sequence is a 1
, the nth term is a n , and the common ratio is
r, then
28
Finding the nth Term of a Geometric Sequence
A) The first term of a geometric sequence is 128,
and the common ratio is 0.5. What is the 10th
term of the sequence?
an a1rn - 1
Write the formula.
a10 (128)(0.5)10 - 1
Substitute 128 for a1 , 10 for n, and 0.5 for r.
a10 (128)(0.5)9
Simplify the exponent.
a10 0.25
Use a calculator.
The 10th term of the sequence is 0.25.
29
Sports Application
A bungee jumper jumps from a bridge. The diagram
shows the bungee jumpers height above the ground
at the top of each bounce. The heights form a
geometric sequence. What is the bungee jumpers
height at the top of the 5th bounce?
The value of r is 0.4.
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an a1rn - 1
Write the formula.
a5 (200)(0.4)5 - 1
Substitute 200 for a1 , 5 for n, and 0.4 for r.
a5 (200)(0.5)4
Simplify the exponent.
a5 5.12
Use a calculator.
The height of the 5th bounce is 5.12 feet.
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You did a great job today!