A geometric sequence is a sequence formed when each consecutive term after the first term, t₁, is multiplied by a constant, with the constant being the common ratio, r.
A geometric sequence increases or decreases at a higher rate than an arithmetic sequence.
An infinite geometric sequence is when the sequence continues on into positive or negative infinity, normally indicated by three periods.
Geometric sequence: 1, 2, 4, 8
Infinite geometric sequence: 1, 2, 4, 8, ...
Common Ratio, r
An infinite geometric sequence can either be convergent or divergent.
If an infinite geometric sequence is convergent, then, as its number of terms is increasing to positive infinity, the term values are converging into a constant value. Generally the convergent value is 0.
If an infinite geometric sequence's term values are not approaching a specific constant, such as zero, then it is not convergent. Instead it is divergent.
If an infinite geometric sequence's term values are fluctuating from positive and negative, then it is not convergent. Instead it is divergent.
Convergent infinite geometric sequence: 0.5, 0.25, 0.125, 0.0625, 0.03125, ... 0
In the sequence, as the number of terms approaches infinity, the term values are approaching the constant, 0.
Divergent infinite geometric sequence: 1, 2, 4, 8, 16, 32, ...
Divergent infinite geometric sequence: 1, -2, 4, -8, 16, -32, ...
Geometric Sequence, tn
tn = t₁rn - 1
tn = general term (mostly the "imaginary" last term of the sequence), said as "nth term"
t₁ = first term
r = common ratio
n = number of terms in the sequence
Finding a Term Value in a Sequence
1. Using the geometric sequence formula, set up the formula with respect to the term needed to be found.
2. Find the common ratio, r, using the geometric sequence and common ratio formula, and find the first term, t₁, by using the initial term in the geometric sequence.
2.1 If the common ratio, r, is not given, set up the geometric sequence formula with respect to a given term other than the first term, t₁, or the term that is needed to be solved, and solve for the common ratio, r, using exponent laws.
2.1.1. After finding two possible values of the common ratio, r, set up two cases for these two possible values and solve for the missing term value.
3. With all variables found, substitute them into the geometric sequence formula and solve for the missing term value.
Finding the Number of Terms in a Sequence
1. Using the geometric sequence formula, set up the formula with respect to the term value given, imagining that that term is the “last term”, tn. If this term value is positive, then the common ratio, r, will be positive too, vice versa.
2. Using exponent laws, solve for the number of terms, n, in the geometric sequence if tn was the last term. If n = a, then tn = tₐ.
PRACTICE QUESTIONS (BOOKLET)
Page: 35
#4, 5, 8
Given the following information:
t₁ = 7 and t₅ = 567
Determine t₆.
1. Using the geometric sequence formula, set up the formula with respect to the term needed to be found.
Since the term needed to be found is t₆, we will set up the formula with respect to t₆.
Setting up the formula, where tn = t₆, n = 6:
tn = t₁rn - 1
t₆ = t₁r6 - 1
t₆ = t₁r⁵
2. Find the common ratio, r, using the geometric sequence and common ratio formula, and find the first term, t₁, by using the initial term in the geometric sequence.
There is no way of finding the common ratio, r, with the given information, so we skip to step 2.1.
2.1 If the common ratio, r, is not given, set up the geometric sequence formula with respect to a given term other than the first term, t₁, or the term that is needed to be solved, and solve for the common ratio, r, using exponent laws.
Since there was no common ratio, r, given, we will use a term given other than the first term, t₁, or the term that is needed to be solved, and solve for the common ratio, r, using exponent laws.
In this case, a given term is t₅ = 567, so we will use this term to find the common ratio, r.
Setting up the formula, where tn = t₅ = 567, n = 5, t₁ = 7:
tn = t₁rn - 1
t₅ = 7r5 - 1
567 = 7r5 - 1
Solving for the common ratio, r, by subtracting 5 and 1:
567 = 7r5 - 1
567 = 7r⁴
Dividing both sides by 7:
567 = 7r⁴
81 = r⁴
Finding the fourth root of both sides:
81 = r⁴
⁴√81 = r
r = ±3
2.1.1. After finding two possible values of the common ratio, r, set up two cases for these two possible values and solve for the missing term value.
We will create two cases for the value of the common ratio, r.
Case 1: t₆ when r = 3
t₆ = t₁r⁵
t₆ = (7)(3)⁵
t₆ = (7)(243)
t₆ = 1701
Case 1: t₆ when r = -3
t₆ = t₁r⁵
t₆ = (7)(-3)⁵
t₆ = (7)(-243)
t₆ = -1701
Therefore, the 6th term, t₆, is either 1701 or -1701, or t₆ = ±1701.
Given the following information:
t₁ = 7 and r = ±3
How many terms are in the geometric sequence if the last term is 45927?
1. Using the geometric sequence formula, set up the formula with respect to the term value given, imagining that that term is the “last term”, tn. If this term value is positive, then the common ratio, r, will be positive too, vice versa.
Since it already states the last term, tn, is 45927, we will use this value and the common ratio, r, to set up the formula.
And since 45927 is a positive number, the common ratio, r, will be positive as well.
Setting up the formula, where tn = 45927, t₁ = 7, r = 3:
tn = t₁rn - 1
45927 = (7)(3)n - 1
2. Using exponent laws, solve for the number of terms, n, in the geometric sequence if tn was the last term. If n = a, then tn = tₐ.
Solving for the number of terms, n, by dividing both sides by 7:
45927 = (7)(3)n - 1
45927 = (3)n - 1
6561 = 3n - 1
Finding the exponent to which 3a = 6561 by guessing and checking:
3a = 3n - 1
a = 8 3⁸ = 6561
Using exponent laws to solve for the number of terms, n, where if ax + y = az + w, then x + y = z + w:
6561 = 3n - 1
3⁸ = 3n - 1
8 = n - 1
n = 9
Therefore, if 45927 was the last term of this geometric sequence, there would be a total of 9 terms, where t₉ = 45927.