Math Problem Statement

The third term of a geometric progression is -108 and the sixth term is 32. Find i) The common ratio and the first term.

ii) Geometric sequence

iii) 10th term

Solution

Given that the third term of a geometric progression (GP) is 108-108 and the sixth term is 3232, we are to find the common ratio, the first term, and other related details.

Step 1: Find the Common Ratio (r)

Let's denote the first term by aa and the common ratio by rr. The nnth term of a geometric sequence can be expressed as:

Tn=arn1T_n = ar^{n-1}

For the third term (T3T_3):

T3=ar31=ar2=108T_3 = ar^{3-1} = ar^2 = -108

For the sixth term (T6T_6):

T6=ar61=ar5=32T_6 = ar^{6-1} = ar^5 = 32

We now have two equations:

ar2=108(Equation 1)ar^2 = -108 \quad \text{(Equation 1)} ar5=32(Equation 2)ar^5 = 32 \quad \text{(Equation 2)}

Step 2: Solve for the Common Ratio (r)

Divide Equation 2 by Equation 1:

ar5ar2=32108\frac{ar^5}{ar^2} = \frac{32}{-108}

Simplifying:

r3=32108=32108=827r^3 = \frac{32}{-108} = \frac{32}{-108} = -\frac{8}{27}

Thus,

r=8273=23r = \sqrt[3]{-\frac{8}{27}} = -\frac{2}{3}

So, the common ratio rr is 23\boxed{-\frac{2}{3}}.

Step 3: Find the First Term (a)

Using the value of rr in Equation 1:

ar2=108ar^2 = -108

Substitute r=23r = -\frac{2}{3}:

a(23)2=108a \left(-\frac{2}{3}\right)^2 = -108

a49=108a \cdot \frac{4}{9} = -108

Multiply both sides by 94\frac{9}{4}:

a=108×94=27×9=243a = -108 \times \frac{9}{4} = -27 \times 9 = -243

So, the first term aa is 243\boxed{-243}.

Step 4: Write the Geometric Sequence

With a=243a = -243 and r=23r = -\frac{2}{3}, the first few terms of the geometric sequence are:

  1. First term: T1=243T_1 = -243
  2. Second term: T2=243×(23)=162T_2 = -243 \times \left(-\frac{2}{3}\right) = 162
  3. Third term: T3=162×(23)=108T_3 = 162 \times \left(-\frac{2}{3}\right) = -108
  4. Fourth term: T4=108×(23)=72T_4 = -108 \times \left(-\frac{2}{3}\right) = 72
  5. Fifth term: T5=72×(23)=48T_5 = 72 \times \left(-\frac{2}{3}\right) = -48
  6. Sixth term: T6=48×(23)=32T_6 = -48 \times \left(-\frac{2}{3}\right) = 32

So, the geometric sequence is:

243,162,108,72,48,32,\boxed{-243, 162, -108, 72, -48, 32, \dots}

Step 5: Find the 10th Term

To find the 10th term (T10T_{10}):

T10=ar101=ar9T_{10} = ar^{10-1} = ar^9

Substitute a=243a = -243 and r=23r = -\frac{2}{3}:

T10=243×(23)9T_{10} = -243 \times \left(-\frac{2}{3}\right)^9

Since (23)9\left(-\frac{2}{3}\right)^9 is negative, we get:

T10=243×51219683=243×51219683=12441619683=6.32(approx)T_{10} = -243 \times -\frac{512}{19683} = \frac{243 \times 512}{19683} = \frac{124416}{19683} = \boxed{6.32} \quad \text{(approx)}

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

  1. What is the general formula for the nnth term of a geometric sequence?
  2. How do you find the sum of the first nn terms in a geometric series?
  3. How would the sequence change if the common ratio was a positive number?
  4. What happens if the common ratio is a fraction greater than 1?
  5. How can you determine if a sequence is geometric based on the terms provided?

Tip: When working with geometric sequences, it's important to consider whether the common ratio is positive or negative, as this will affect the sign of the terms in the sequence.

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Math Problem Analysis

Mathematical Concepts

Geometric Progression
Common Ratio
Nth Term Formula

Formulas

Nth term of a geometric sequence formula: T_n = a * r^(n-1)

Theorems

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Suitable Grade Level

Grades 9-12