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8. Calculus
Using the formula for the sum to infinity of a convergent geometric progression

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Using the Formula for the Sum to Infinity of a Convergent Geometric Progression

Introduction

Understanding the sum to infinity of a convergent geometric progression is fundamental in mathematics, particularly within the Cambridge IGCSE curriculum for Mathematics - Additional - 0606. This concept not only reinforces students' grasp of series and sequences but also serves as a foundational tool in various real-world applications, including finance, physics, and engineering.

Key Concepts

Geometric Progression: Definition and Basic Properties

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 is the first term.
  • r is the common ratio.

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.

Sum of a Finite Geometric Progression

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:

  • n is the number of terms.
  • a is the first term.
  • r is the common ratio.

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 \( S_n \).

Sum to Infinity of a Convergent Geometric Progression

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.

Derivation of the Sum to Infinity Formula

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.

Examples and Applications

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.

Conditions for Convergence

For a geometric progression to be convergent and for the sum to infinity to exist:

  • The common ratio must satisfy \( |r| < 1 \).
  • The first term \( a \) must be finite.

If \( |r| \geq 1 \), the GP does not converge, and the sum to infinity is not defined.

Real-World Applications

The sum to infinity of a convergent geometric progression finds applications in various fields:

  • Finance: Calculating the present value of perpetuities, which are infinite series of cash flows.
  • Physics: Analyzing phenomena that involve diminishing quantities, such as radioactive decay.
  • Engineering: Designing systems with feedback loops that attenuate over time.

Graphical Interpretation

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.

Common Misconceptions

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.

Practice Problems

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.

Advanced Concepts

Theoretical Foundations and Mathematical Proofs

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.

Mathematical Derivations

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.

Complex Problem-Solving

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.

Interdisciplinary Connections

The concept of the sum to infinity of a convergent geometric progression bridges several disciplines:

  • Economics: Modeling perpetual growth or decay scenarios, such as population dynamics or investment returns.
  • Computer Science: Analyzing algorithmic efficiency, particularly in recursive algorithms where each step reduces the problem size by a constant factor.
  • Environmental Science: Predicting the long-term impact of sustainable practices or resource usage.

Extension to Real Numbers and Complex Analysis

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.

Applications in Financial Mathematics

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.

Exploring Series Convergence Criteria

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.

Multiple Summation Techniques

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.

Advanced Practice Problems

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.

Comparison Table

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

Summary and Key Takeaways

  • The sum to infinity formula \( S_\infty = \frac{a}{1 - r} \) applies only when \( |r| < 1 \).
  • Geometric progressions are pivotal in various real-world applications, including finance and physics.
  • Understanding convergence criteria is essential for analyzing infinite series.
  • Advanced problem-solving techniques enhance comprehension and application of infinite sums.

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Examiner Tip
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Tips

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:

  • Convergence requires \( |r| < 1 \)
  • Ratio must be less than one in absolute value
  • Infinite sums need a bounded ratio
  • Streetlight: Only diminishing terms ensure convergence
  • Simple formula: \( S_\infty = \frac{a}{1 - r} \)
Additionally, practice deriving the sum to infinity formula from the finite sum to reinforce your understanding.

Did You Know
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Did You Know

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.

Common Mistakes
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Common Mistakes

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 \( S_n = a \cdot \frac{1 - r^n}{1 - r} \).

FAQ

What is a geometric progression?
A geometric progression is a sequence of numbers where each term after the first is multiplied by a constant ratio, \( r \).
When does a geometric series converge?
A geometric series converges when the absolute value of the common ratio \( r \) is less than 1 (\( |r| < 1 \)).
How do you calculate the sum to infinity of a convergent geometric series?
Use the formula \( S_\infty = \frac{a}{1 - r} \) where \( a \) is the first term and \( r \) is the common ratio.
Can the sum to infinity be negative?
Yes, if the first term \( a \) is negative and the common ratio \( r \) satisfies \( |r| < 1 \).
What happens if \( |r| = 1 \) in a geometric series?
If \( |r| = 1 \), the geometric series does not converge, and the sum to infinity is undefined.
8. Calculus
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