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Solution - Geometric Sequences

The common ratio is:

r = 11

r=11

The sum of this series is:

s = 16104

s=16104

The general form of this series is:

a_{n} = 11 \cdot 11^{n - 1}

a_n=11*11^(n-1)

The nth term of this series is:

11, 121, 1331, 14641, 161051, 1771561, 19487171, 214358881, 2357947691, 25937424601

11,121,1331,14641,161051,1771561,19487171,214358881,2357947691,25937424601

See steps

Step-by-step explanation

1. Find the common ratio

Find the common ratio by dividing any term in the sequence by the term that comes before it:

$a_{2} a_{1} = \frac{121}{11} = 11$

$a_{3} a_{2} = \frac{1331}{121} = 11$

$a_{4} a_{3} = \frac{14641}{1331} = 11$

The common ratio ( $r$ ) of the sequence is constant and equals the quotient of two consecutive terms.
$r = 11$

2. Find the sum

5 additional steps

$s_{n} = a * ((1 - r^{n}) / (1 - r))$

To find the sum of the series, plug the first term: $a = 11$ , the common ratio: $r = 11$ , and the number of elements $n = 4$ into the geometric series sum formula:

$s_{4} = 11 * ((1 - 11^{4}) / (1 - 11))$

$s_{4} = 11 * ((1 - 14641) / (1 - 11))$

$s_{4} = 11 * (- 14640 / (1 - 11))$

$s_{4} = 11 * (- 14640 / - 10)$

$s_{4} = 11 \cdot 1464$

$s_{4} = 16104$

3. Find the general form

$a_{n} = a \cdot r^{n - 1}$

To find the general form of the series, plug the first term: $a = 11$ and the common ratio: $r = 11$ into the formula for geometric series:

$a_{n} = 11 \cdot 11^{n - 1}$

4. Find the nth term

Use the general form to find the nth term

$a_{1} = 11$

$a_{2} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{2 - 1} = 11 \cdot 11^{1} = 11 \cdot 11 = 121$

$a_{3} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{3 - 1} = 11 \cdot 11^{2} = 11 \cdot 121 = 1331$

$a_{4} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{4 - 1} = 11 \cdot 11^{3} = 11 \cdot 1331 = 14641$

$a_{5} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{5 - 1} = 11 \cdot 11^{4} = 11 \cdot 14641 = 161051$

$a_{6} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{6 - 1} = 11 \cdot 11^{5} = 11 \cdot 161051 = 1771561$

$a_{7} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{7 - 1} = 11 \cdot 11^{6} = 11 \cdot 1771561 = 19487171$

$a_{8} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{8 - 1} = 11 \cdot 11^{7} = 11 \cdot 19487171 = 214358881$

$a_{9} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{9 - 1} = 11 \cdot 11^{8} = 11 \cdot 214358881 = 2357947691$

$a_{10} = a_{1} \cdot r^{n - 1} = 11 \cdot 11^{10 - 1} = 11 \cdot 11^{9} = 11 \cdot 2357947691 = 25937424601$

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Why learn this

Geometric sequences are commonly used to explain concepts in mathematics, physics, engineering, biology, economics, computer science, finance, and more, making them a very useful tool to have in our toolkits. One of the most common applications of geometric sequences, for example, is calculating earned or unpaid compound interest, an activity most commonly associated with finance that could mean earning or losing a lot of money! Other applications include, but are certainly not limited to, calculating probability, measuring radioactivity over time, and designing buildings.

Terms and topics

Geometric Sequences