Math Problem Statement

Determine an expression for the nth term of the following sequence if the: a) 4th term is 24 and the 7th term is 192 in a geometric sequence. c) 6th term is 13 and the 14th term is 33 in an arithmetic sequence.

Solution

Let's analyze the problem step by step.

Part (a): Geometric Sequence

We are given:

  • The 4th term (a4a_4) = 24
  • The 7th term (a7a_7) = 192

General formula for a geometric sequence:

an=arn1a_n = a \cdot r^{n-1} where:

  • aa = first term
  • rr = common ratio

From the formula: a4=ar3=24(1)a_4 = a \cdot r^3 = 24 \quad \text{(1)} a7=ar6=192(2)a_7 = a \cdot r^6 = 192 \quad \text{(2)}

Solve for rr:

Dividing equation (2) by equation (1): ar6ar3=19224\frac{a \cdot r^6}{a \cdot r^3} = \frac{192}{24} r3=8    r=2r^3 = 8 \quad \implies \quad r = 2

Solve for aa:

Substitute r=2r = 2 into equation (1): a23=24a \cdot 2^3 = 24 a8=24    a=3a \cdot 8 = 24 \quad \implies \quad a = 3

Final expression for ana_n:

an=32n1a_n = 3 \cdot 2^{n-1}


Part (c): Arithmetic Sequence

We are given:

  • The 6th term (a6a_6) = 13
  • The 14th term (a14a_{14}) = 33

General formula for an arithmetic sequence:

an=a+(n1)da_n = a + (n-1)d where:

  • aa = first term
  • dd = common difference

From the formula: a6=a+5d=13(1)a_6 = a + 5d = 13 \quad \text{(1)} a14=a+13d=33(2)a_{14} = a + 13d = 33 \quad \text{(2)}

Solve for dd:

Subtract equation (1) from equation (2): (a+13d)(a+5d)=3313(a + 13d) - (a + 5d) = 33 - 13 8d=20    d=2.58d = 20 \quad \implies \quad d = 2.5

Solve for aa:

Substitute d=2.5d = 2.5 into equation (1): a+5(2.5)=13a + 5(2.5) = 13 a+12.5=13    a=0.5a + 12.5 = 13 \quad \implies \quad a = 0.5

Final expression for ana_n:

an=0.5+(n1)(2.5)a_n = 0.5 + (n-1)(2.5) Simplify: an=0.5+2.5n2.5a_n = 0.5 + 2.5n - 2.5 an=2.5n2a_n = 2.5n - 2


Final Answers:

(a) For the geometric sequence: an=32n1a_n = 3 \cdot 2^{n-1}

(c) For the arithmetic sequence: an=2.5n2a_n = 2.5n - 2

Would you like further clarification or a deeper explanation for any part? Here are 5 related questions to explore further:

  1. How can we determine the nth term of a sequence from other given terms?
  2. What is the difference between geometric and arithmetic sequences?
  3. How can the common ratio of a geometric sequence be negative, and how does that affect the sequence?
  4. What happens when you calculate a fractional or decimal rr in a geometric sequence?
  5. Can we derive the sum of an arithmetic sequence given the formula for its nth term?

Tip: Always double-check calculations by substituting the values back into the sequence formulas to verify consistency!

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Math Problem Analysis

Mathematical Concepts

Geometric Sequences
Arithmetic Sequences
Algebra

Formulas

Geometric sequence: a_n = a * r^(n-1)
Arithmetic sequence: a_n = a + (n-1)d

Theorems

Geometric sequence properties
Arithmetic sequence properties

Suitable Grade Level

Grades 9-12