Math Problem Statement

The first term and the sum of all terms for a geometric sequence is 4 and -59048 respectively. Given that the common ratio of the sequence is -3, find the number of terms of this geometric sequence.

Solution

This problem involves solving for the number of terms in a geometric sequence. Let's solve it step by step:

Given:

  1. The first term (aa) is 4.
  2. The sum of all terms (SnS_n) is 59048-59048.
  3. The common ratio (rr) is 3-3.
  4. The formula for the sum of a geometric sequence: Sn=a1rn1r,r1.S_n = a \frac{1 - r^n}{1 - r}, \quad r \neq 1.

Step 1: Substitute known values

Using the formula for the sum of the sequence: 59048=41(3)n1(3).-59048 = 4 \cdot \frac{1 - (-3)^n}{1 - (-3)}.

Simplify the denominator: 59048=41(3)n4.-59048 = 4 \cdot \frac{1 - (-3)^n}{4}.

Cancel the 4: 59048=1(3)n.-59048 = 1 - (-3)^n.

Rearrange to isolate (3)n(-3)^n: (3)n=1+59048.(-3)^n = 1 + 59048.

(3)n=59049.(-3)^n = 59049.

Step 2: Solve for nn

Take the logarithm of both sides or solve by inspection: (3)n=310.(-3)^n = 3^{10}.

Since the base is 3-3, nn must be 10 because the powers of (3)(-3) alternate signs. Here, the result matches the positive side.

Final Answer:

n=10.n = 10.

Let me know if you’d like further clarification or verification!


Related Questions:

  1. How would the solution change if the common ratio were positive?
  2. Can this method be generalized for other common ratios?
  3. What happens if the sum SnS_n is positive but rr is negative?
  4. How would the formula change for an infinite geometric sequence?
  5. What practical applications use geometric sequences with negative ratios?

Tip:

When solving equations involving powers, logarithms can simplify the process—especially when nn isn't an integer.

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Math Problem Analysis

Mathematical Concepts

Geometric Sequences
Exponential Equations

Formulas

Sum of a finite geometric sequence Sn = a(1 - r^n) / (1 - r) for r ≠ 1

Theorems

Laws of Exponents

Suitable Grade Level

Grades 10-12