Math Problem Statement

a sequence of numbers follows the rule: multiply the previous number by -2 and add 3, the fourth term in the sequence is -7

Solution

Let's determine the first three terms of the sequence using the rule: multiply the previous number by 2-2 and add 33.

Let the first term be a1a_1, the second term be a2a_2, and so on. We know:

a4=7a_4 = -7

To find the terms that came before a4=7a_4 = -7, we work backwards:

  1. Find a3a_3:
    a4=2×a3+3a_4 = -2 \times a_3 + 3 Substitute a4=7a_4 = -7: 7=2×a3+3-7 = -2 \times a_3 + 3 Solve for a3a_3: 10=2×a3    a3=5-10 = -2 \times a_3 \implies a_3 = 5

  2. Find a2a_2:
    a3=2×a2+3a_3 = -2 \times a_2 + 3 Substitute a3=5a_3 = 5: 5=2×a2+35 = -2 \times a_2 + 3 Solve for a2a_2: 2=2×a2    a2=12 = -2 \times a_2 \implies a_2 = -1

  3. Find a1a_1:
    a2=2×a1+3a_2 = -2 \times a_1 + 3 Substitute a2=1a_2 = -1: 1=2×a1+3-1 = -2 \times a_1 + 3 Solve for a1a_1: 4=2×a1    a1=2-4 = -2 \times a_1 \implies a_1 = 2

So, the first three terms in the sequence are:

  1. a1=2a_1 = 2
  2. a2=1a_2 = -1
  3. a3=5a_3 = 5

The three terms that come before 7-7 are 2, -1, and 5.

Would you like more details or have any questions? Here are some related questions:

  1. What is the fifth term in this sequence?
  2. How do you find the general formula for the nnth term in such sequences?
  3. How does changing the multiplier from 2-2 to another number affect the sequence?
  4. What if the rule was "divide by 2 and subtract 3" instead?
  5. Can you find the sum of the first 10 terms in this sequence?

Tip: Always verify your final answers by plugging them back into the original rule to ensure consistency.

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Math Problem Analysis

Mathematical Concepts

Arithmetic Sequences

Formulas

General formula for arithmetic sequences

Theorems

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Suitable Grade Level

Grades 7-9