Somme d'une série géométrique : Comprendre la formule et ses applications

The Sum of a Geometric Series: An Easy Guide

Calculating the sum of a geometric series might sound complex, but let’s break it down together in a way that is both engaging and straightforward. Imagine you have a set of numbers where each number is a constant multiple of the previous one. This set of numbers forms what we call a geometric series.

Understanding the Formula

The sum of the first n terms of a geometric series is given by the formula:

Let’s dissect this formula to understand it better:

• a - The first term of the geometric series.
• r - The common ratio (the factor you multiply each term by to get the next term). This ratio is unit-less, meaning it's neither meters nor dollars, just a pure number.
• n - The number of terms. This is a positive integer (e.g., 1, 2, 3).

The output S represents the sum of the first n terms of the series.

Real-Life Example

Consider a scenario where you deposit $1,000 in the first year into a savings account that promises a yearly interest rate of 5%. Assuming you deposit the same amount each year but each year’s deposit grows by 5% from the previous year’s amount saved, calculating the total savings after 3 years would represent the sum of a geometric series. Here's how you can apply the formula:

Parameters:

• First term a= 1000 (USD)
• Common ratio r = 1.05
• Number of terms n = 3 years

By plugging these into our formula:

Hence, after 3 years, your total savings would be approximately $3,152.50 USD.

Deeper Into the Series

As exciting as geometric series are, the magic comes to life when we delve into the sequence’s behavior as the number of terms increases. If the common ratio r lies between -1 and 1 (excluding 1 itself), the sum of an infinite geometric series simplifies to:

This formula holds true because as n approaches infinity, r^n approaches zero.

Practical Applications

Geometric series are not just theoretical; they are practical tools used in varied domains including finance, computer science, and physics. For instance, in finance, calculating the present value of an annuity employs the concept of geometric series.

Exploring More Examples

Let’s say you want to determine the total distance a ball travels before coming to rest, if it bounces back to 50% of its previous height after each bounce. If the ball is dropped from an initial height of 2 meters, the series formed by the distances will be a geometric series where a= 2 meters, r= 0.5, and each term represents the distance traveled in one bounce.

Using the formula:

The total distance traveled by the ball will be 4 meters before it comes to rest.

Summary

The sum of a geometric series formula is not just a handy mathematical tool; it’s something you can apply in countless real-world situations. It’s powerful yet simple enough to grasp with just a bit of understanding. By knowing the first term, the common ratio, and the number of terms, you can unlock significant insights into growth patterns, saving calculations, and even physical phenomena.