Maîtriser le nième terme d'une suite géométrique : dévoiler la formule

Understanding Geometric Sequence and Its nth Term

Geometric sequence is a fascinating concept in algebra that many students encounter during their mathematics journey. Simply put, a geometric sequence is a list of numbers where each term after the first is found by multiplying the previous term by a non-zero number called the common ratio.

Importance of Geometric Sequences

Geometric sequences are not just abstract mathematical ideas but have real applications in finance, biology, and computer science. Understanding the formula for the nth term of a geometric sequence can help you predict values without needing to manually multiply every term.

The Geometric Sequence nth Term Formula

The formula to determine the nth term of a geometric sequence is:

an = a1 × r(n-1)

Where:

• an = nth term of the sequence
• a1 = first term of the sequence
• r = common ratio (must be a non-zero number)
• n = position of the term (must be a positive integer)

Breaking Down the Formula

Let's dive deeper into each component of the formula:

• First Term (a1): The starting point of the sequence. For example, in a sequence starting with 3, a1 is 3.
• Common Ratio (r): This is the multiplier used to get from one term to the next. If each number is doubled, then r is 2. If each term is halved, r is 0.5.
• Position (n): This indicates which term you want to find in the sequence. If you need the 5th term, n is 5.

Real-Life Examples of Geometric Sequence

Example 1: Biological Growth

Imagine a bacteria culture that doubles every hour. If the initial population is 100 bacteria, you can use the formula to find the number of bacteria after 5 hours:

• a1 = 100
• r = 2
• n = 6 (because we start at hour 0)

The number of bacteria after 5 hours is:

a6 = 100 × 2(6-1) = 100 × 25 = 100 × 32 = 3200

Example 2: Finance

Suppose you invest $1,000 in a fund that grows at a rate of 5% per year. To find out how much you'd have after 10 years, you can set it up as follows:

• a1 = 1000
• r = 1.05
• n = 11 (including the initial investment year)

The amount after 10 years is:

a11 = 1000 × 1.05(11-1) = 1000 × 1.0510 = 1000 × 1.62889 ≈ 1628.89 USD

Validation of the Formula

Ensuring your values make sense is crucial. Here are guidelines:

• a1: Can be any real number.
• r: Should not be zero.
• n: Must be a positive integer.

Frequently Asked Questions

Q: What happens if the common ratio is 1?

A: If r=1, every term in the sequence is the same as the first term.

Q: Can the common ratio be negative?

A: Yes, a negative common ratio will result in the terms alternating between positive and negative values.

Q: What if I need to find a term in a sequence starting with decimal values?

A: The formula works just as well for decimal and fractional values.

Conclusion

Geometric sequences offer an elegant way to describe patterns and predict future values. Whether it's predicting population growth or calculating potential investment returns, this formula provides an accessible pathway to deriving meaningful insights.