Geometric Sequences and Their Application

Geometric Sequences

• Definition: A geometric sequence is a sequence of numbers where each term after the first is found by multiplying the previous term by a fixed, non-zero number called the common ratio.

• Daily Objective: Students will be able to use the geometric sequences formula to find the nth term.

Key Characteristics of Sequences

• Arithmetic Sequence:

  • Defined as a sequence where the difference between consecutive terms is constant.
  • Example: 5, 10, 15 (common difference = +5)
  • Example: 12, 6, 0, -6, -12 (common difference = -6)

• Geometric Sequence:

  • Characterized by the ratio of consecutive terms being constant. The ratio is known as the common ratio.
  • Example: 2, 4, 8, 16, 32 (here the common ratio is 2, each term is multiplied by 2)
  • This does not qualify as an arithmetic sequence because it does not have a constant difference.

Formulas to Know

• Geometric Sequence Formula: an = a1 imes r^{(n-1)}
  • Where:
  • a_n is the nth term of the sequence,
  • a_1 is the first term,
  • r is the common ratio,
  • n is the term number.

Example Problem

• Find the 13th term in the sequence: 4, 12, 36, …
  • Identify the first term and common ratio:
  • a_1 = 4
  • Sequence pattern: 4, 12, 36 (after examining, multiply by 3 to get from 4 to 12, and from 12 to 36)
  • Common Ratio r = 3
  • Using the formula:
    a_n = 4 imes 3^{(n-1)}
  • To find the 13th term:
    a_{13} = 4 imes 3^{(13-1)} = 4 imes 3^{12}

Conclusion

• A clear understanding of geometric and arithmetic sequences is essential, especially in recognizing their defining characteristics (common difference vs. common ratio).
• Apply the appropriate formulas to find terms in these sequences, facilitating problem-solving in related mathematical contexts.