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Topic 2/3
15 Flashcards in this deck.
A geometric progression (GP) is a sequence of numbers where each term after the first is found by multiplying the previous term by a constant ratio, denoted as \( r \). Mathematically, a GP can be expressed as:
\( a, ar, ar^2, ar^3, \ldots \)
where:
A GP is called convergent if the absolute value of the common ratio is less than 1, i.e., \( |r| < 1 \). This implies that as the progression continues, the terms get closer to zero.
Before delving into the sum to infinity, it's essential to understand the sum of the first \( n \) terms of a GP, denoted as \( S_n \). The formula for \( S_n \) is:
$$ S_n = a \cdot \frac{1 - r^n}{1 - r} $$where:
This formula is derived by considering the sum \( S_n \) and multiplying it by \( r \), then subtracting the two equations to eliminate most terms and solve for \( S_n \).
When the number of terms in a GP approaches infinity (\( n \to \infty \)), and the GP is convergent (\( |r| < 1 \)), the sum to infinity (\( S_\infty \)) can be calculated using the formula:
$$ S_\infty = \frac{a}{1 - r} $$This formula simplifies the process of adding an infinite number of terms in a convergent GP by leveraging the fact that the successive terms become negligible as \( n \) increases.
To derive the sum to infinity formula, start with the finite sum formula:
$$ S_n = a \cdot \frac{1 - r^n}{1 - r} $$As \( n \to \infty \) and \( |r| < 1 \), \( r^n \to 0 \). Thus:
$$ S_\infty = a \cdot \frac{1 - 0}{1 - r} = \frac{a}{1 - r} $$This derivation highlights the conditions under which the sum to infinity is valid, emphasizing the importance of the common ratio's absolute value being less than one.
Consider the following example to illustrate the application of the sum to infinity formula:
Example: Find the sum to infinity of the GP where \( a = 5 \) and \( r = \frac{1}{3} \).
Solution:
$$ S_\infty = \frac{5}{1 - \frac{1}{3}} = \frac{5}{\frac{2}{3}} = 5 \times \frac{3}{2} = \frac{15}{2} = 7.5 $$Therefore, the sum to infinity is 7.5.
For a geometric progression to be convergent and for the sum to infinity to exist:
If \( |r| \geq 1 \), the GP does not converge, and the sum to infinity is not defined.
The sum to infinity of a convergent geometric progression finds applications in various fields:
Graphically, a convergent GP can be represented as a decreasing function approaching zero. As more terms are added, the value of the sum \( S_n \) approaches \( S_\infty \), illustrating the concept of convergence visually.
A prevalent misconception is that the sum to infinity of any geometric progression can be calculated using \( S_\infty = \frac{a}{1 - r} \). However, this is only valid when \( |r| < 1 \). If \( |r| \geq 1 \), the series diverges, and the sum to infinity does not exist.
Problem 1: Find the sum to infinity of the GP with first term 8 and common ratio \( \frac{1}{2} \).
Solution: $$ S_\infty = \frac{8}{1 - \frac{1}{2}} = \frac{8}{\frac{1}{2}} = 16 $$
Problem 2: Determine whether the GP with \( a = 10 \) and \( r = 1.2 \) is convergent. If convergent, find the sum to infinity.
Solution: Since \( |r| = 1.2 > 1 \), the GP is divergent. Therefore, the sum to infinity does not exist.
Delving deeper into the sum to infinity of a convergent geometric progression involves exploring its theoretical underpinnings. To comprehend why the series converges when \( |r| < 1 \), consider the limit of the partial sums as \( n \) approaches infinity.
Starting with the finite sum formula:
$$ S_n = a \cdot \frac{1 - r^n}{1 - r} $$We analyze the behavior of \( r^n \) as \( n \to \infty \). Since \( |r| < 1 \), \( r^n \) diminishes exponentially towards zero. Thus:
$$ \lim_{{n \to \infty}} S_n = \frac{a}{1 - r} $$This limit exists and is finite, confirming the convergence of the series.
Expanding the derivation of the infinite sum, consider the infinite series:
$$ S_\infty = a + ar + ar^2 + ar^3 + \ldots $$Multiply both sides by \( r \):
$$ rS_\infty = ar + ar^2 + ar^3 + ar^4 + \ldots $$>Subtract the second equation from the first:
$$ S_\infty - rS_\infty = a $$> $$ S_\infty(1 - r) = a $$> $$ S_\infty = \frac{a}{1 - r} $$>This derivation succinctly demonstrates the relationship between the infinite sum and the common ratio.
Consider a scenario where a GP represents the decay of a radioactive substance. Suppose the initial quantity of the substance is 100 grams, and it decays by 20% each hour. Determine the total quantity decayed over an infinite time period.
Solution:
Here, \( a = 100 \) grams and \( r = 0.8 \) (since the substance retains 80% each hour, losing 20%). The GP is convergent because \( |r| = 0.8 < 1 \).
$$ S_\infty = \frac{100}{1 - 0.8} = \frac{100}{0.2} = 500 \text{ grams} $$>Thus, a total of 500 grams will have been decayed over an infinite time period.
The concept of the sum to infinity of a convergent geometric progression bridges several disciplines:
Extending the sum to infinity formula beyond real numbers, in the realm of complex analysis, the convergence criteria remain similar. For a geometric series with a complex common ratio \( r \), convergence requires \( |r| < 1 \). This extension is pivotal in fields like electrical engineering, where alternating current (AC) circuits may involve complex impedances.
In financial mathematics, the sum to infinity formula is instrumental in calculating the present value of perpetuities—financial instruments that pay a steady stream of cash flows indefinitely. For example, a perpetuity paying \( P \) dollars annually with a discount rate \( d \) can be valued as:
$$ PV = \frac{P}{d} $$>This application showcases the practical significance of the geometric progression in real-world financial scenarios.
Beyond geometric series, understanding convergence criteria is essential in series analysis. The geometric series serves as a fundamental example illustrating the importance of the common ratio's magnitude in determining convergence, thereby laying the groundwork for exploring more complex series in calculus and analysis.
Advanced summation techniques, such as telescoping series or using generating functions, can be employed to evaluate more intricate infinite series. While the geometric series offers a straightforward sum to infinity formula, these techniques expand the toolkit available for tackling diverse mathematical challenges.
Problem 3: A perpetuity pays \$200 annually. If the discount rate is 5%, calculate the present value of the perpetuity using the sum to infinity formula.
Solution: $$ PV = \frac{200}{0.05} = 4000 $$>
Therefore, the present value is \$4,000.
Problem 4: Prove that the sum to infinity of a geometric series converges only if \( |r| < 1 \) using the finite sum formula.
Solution: Starting with: $$ S_n = a \cdot \frac{1 - r^n}{1 - r} $$>
For \( S_\infty \) to exist: $$ \lim_{{n \to \infty}} S_n = \frac{a}{1 - r} $$>
This limit is finite only if \( \lim_{{n \to \infty}} r^n = 0 \), which holds true when \( |r| < 1 \). If \( |r| \geq 1 \), \( r^n \) does not approach zero, making \( S_\infty \) undefined.
Aspect | Finite Geometric Progression | Infinite Geometric Progression |
---|---|---|
Number of Terms | Finite (\( n \) | Infinite (\( n \to \infty \)) |
Sum Formula | \( S_n = a \cdot \frac{1 - r^n}{1 - r} \) | \( S_\infty = \frac{a}{1 - r} \) |
Convergence Condition | Always converges for finite \( n \) | Converges only if \( |r| < 1 \) |
Applications | Calculating total amounts over a specific number of periods | Modeling perpetuities, decaying processes |
Behavior as \( n \) Increases | Sum increases with \( n \) | Sum approaches a finite limit |
To easily remember the convergence condition, think of the common ratio \( r \) as a "rate of return" that must be contained within a boundary; if it exceeds 1, the series escapes to infinity. Use the mnemonic "CRISS" to recall:
Did you know that the concept of the sum to infinity of a geometric progression is fundamental in calculating the value of perpetuities in finance? For instance, the present value of a perpetual bond can be determined using this formula. Additionally, this mathematical principle was pivotal in the early development of calculus, influencing mathematicians like Isaac Newton and Gottfried Wilhelm Leibniz in their work on infinite series.
One common mistake students make is forgetting to check if the common ratio \( r \) satisfies \( |r| < 1 \) before applying the sum to infinity formula. For example, using \( S_\infty = \frac{a}{1 - r} \) when \( r = 1.2 \) leads to an incorrect conclusion because the series does not converge. Another frequent error is incorrectly simplifying the finite sum formula by misapplying the exponent, such as writing \( S_n = a \cdot \frac{1 - r^n}{1 + r} \) instead of \( S_n = a \cdot \frac{1 - r^n}{1 - r} \).