2-1 arithmetic and geometric sequences
TRANSCRIPT
Unit 2 Sequences and Series
2‐1 Arithmetic and Geometric SequencesIdentifying sequencesTerm values
2‐2 Arithmetic and Geometric SeriesPartial sumsConvergent geometric series
2‐3 Factorials and the Binomial FormulaSimplify factorialsBinomial series
2‐1 Sequences
Unit 2 Sequences and Series
Concepts and Objectives
Sequences (Obj. #6)Given the first few terms of a sequence, tell whether it is arithmetic, geometric, or neitherFind the nth term of a sequenceFind the index of a given term of a sequence
Sequences
A sequence is a function whose domain is the set of natural numbers ( ) (the term numbers), and whose range is the set of term values.
Examples: Find a next term for the following:A. 3, 10, 17, 24, 31, …
B. 3, 6, 12, 24, 48, …
Sequences
A sequence is a function whose domain is the set of natural numbers ( ) (the term numbers), and whose range is the set of term values.
Examples: Find a next term for the following:A. 3, 10, 17, 24, 31, …
38 (add 7)B. 3, 6, 12, 24, 48, …
96 (multiply by 2)
Sequences
An arithmetic sequence is a sequence in which one term equals a constant added to the preceding term.
The constant for an arithmetic sequence is called the common difference, d, because the difference between any two adjacent terms equals this constant.
A geometric sequence is a sequence in which each term equals a constant multiplied by the preceding term.
The constant for a geometric sequence is called the common ratio, r, because the ratio between any two adjacent terms equals this constant.
Sequences
Example: Is the sequence 4, 7, 10, … arithmetic, geometric, or neither?
Sequences
Example: Is the sequence 4, 7, 10, … arithmetic, geometric, or neither?
Solution: Check to see if adjacent terms have a common difference or a common ratio:7 – 4 = 3 10 – 7 = 3 common difference
no common ratio
The sequence is arithmetic.
10 77 4≠
Sequences
Formulas for calculating tn for arithmetic and geometric sequences can be found by linking the term number to the term value.Example: The arithmetic sequence 3, 10, 17, 24, 31, …, has a first term t1 = 3, and common difference d = 7:
Sequences
Formulas for calculating tn for arithmetic and geometric sequences can be found by linking the term number to the term value.Example: The arithmetic sequence 3, 10, 17, 24, 31, …, has a first term t1 = 3, and common difference d = 7:
1 3t =
2 3 7t = +
( )( )3 3 7 7 3 2 7t = + + = +
( )( )4 3 7 7 7 3 3 7t = + + + = +
( )( )3 1 7nt n= + −
Sequences
The nth term of an arithmetic sequence equals the first term plus (n – 1) common differences. That is,
We can see that this is a linear function (where n is the independent variable), and if we compare this to the slope‐intercept form, we can see that the slope is d, and the y‐intercept would be t0, or t1 – d if zero were in the domain of the function (remember that our domain is ).
( )1 1nt t n d= + −
Sequences
Similarly, we can construct a geometric sequence:3, 6, 12, 24, 48, …
Sequences
Similarly, we can construct a geometric sequence:3, 6, 12, 24, 48, …
This sequence has t1 = 3 and common ratio r = 2. Thus:
1 3t =
2 3 2t = i2
3 3 2 2 3 2t = =i i i3
4 3 2 2 2 3 2t = =i i i i13 2nnt−= i
Sequences
The nth term of a geometric sequence equals the first term multiplied by (n – 1) common ratios. That is,
A geometric sequence is actually just an example of an exponential function. The only difference is that the domain of a geometric sequence is rather than all real numbers.
11
nnt t r −=
Sequences ‐ Examples
1. Calculate t100 for the arithmetic sequence17, 22, 27, 32, …
2. Calculate t100 for the geometric sequence with first term t1 = 35 and common ratio r = 1.05.
Sequences ‐ Examples
1. Calculate t100 for the arithmetic sequence17, 22, 27, 32, …
d = 5Therefore,
2. Calculate t100 for the geometric sequence with first term t1 = 35 and common ratio r = 1.05.
( )( )100 17 100 1 5t = + −17 495 512= + =
( )( )100 1100 35 1.05t −=
( )( )9935 1.05 4383.375262= =
Sequences ‐ Examples
3. The number 68 is a term in the arithmetic sequence with t1 = 5 and d = 3. Which term is it?
Sequences ‐ Examples
3. The number 68 is a term in the arithmetic sequence with t1 = 5 and d = 3. Which term is it?
( )( )68 5 1 3n= + −
( )( )63 1 3n= −
21 1n= −22n =
Sequences ‐ Examples
4. A geometric sequence has t1 = 17 and r = 2. If tn = 34816, find n.
Sequences ‐ Examples
4. A geometric sequence has t1 = 17 and r = 2. If tn = 34816, find n.
To solve for n, we will take the log of each side:
( )( )134816 17 2n−=12048 2n−=
1log2048 log2n−=( )log2048 1 log2n= −
log2048 1log2
n= −
11 1n= −12n =