Algebra Examples

$1 + \frac{1}{3} + \frac{1}{9} + \frac{1}{27}$

Step 1

This is a geometric sequence since there is a common ratio between each term. In this case, multiplying the previous term in the sequence by

$\frac{1}{3}$ gives the next term. In other words,

$a_{n} = a_{1} r^{n - 1}$ .

Geometric Sequence:

$r = \frac{1}{3}$

Step 2

This is the form of a geometric sequence.

$a_{n} = a_{1} r^{n - 1}$

Step 3

Substitute in the values of

$a_{1} = 1$ and

$r = \frac{1}{3}$ .

$a_{n} = 1 {(\frac{1}{3})}^{n - 1}$

Step 4

Multiply

${(\frac{1}{3})}^{n - 1}$ by

$1$ .

$a_{n} = {(\frac{1}{3})}^{n - 1}$

Step 5

Apply the product rule to

$\frac{1}{3}$ .

$a_{n} = \frac{1^{n - 1}}{3^{n - 1}}$

Step 6

One to any power is one.

$a_{n} = \frac{1}{3^{n - 1}}$

Step 7

This is the formula to find the sum of the first

$n$ terms of the geometric sequence. To evaluate it, find the values of

$r$ and

$a_{1}$ .

$S_{n} = \frac{a_{1} (r^{n} - 1)}{r - 1}$

Step 8

Replace the variables with the known values to find

$S_{4}$ .

$S_{4} = 1 \cdot \frac{{(\frac{1}{3})}^{4} - 1}{\frac{1}{3} - 1}$

Step 9

Multiply

$\frac{{(\frac{1}{3})}^{4} - 1}{\frac{1}{3} - 1}$ by

$1$ .

$S_{4} = \frac{{(\frac{1}{3})}^{4} - 1}{\frac{1}{3} - 1}$

Step 10

Multiply the numerator and denominator of the fraction by

$3$ .

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Step 10.1

Multiply

$\frac{{(\frac{1}{3})}^{4} - 1}{\frac{1}{3} - 1}$ by

$\frac{3}{3}$ .

$S_{4} = \frac{3}{3} \cdot \frac{{(\frac{1}{3})}^{4} - 1}{\frac{1}{3} - 1}$

Step 10.2

Combine.

$S_{4} = \frac{3 ({(\frac{1}{3})}^{4} - 1)}{3 (\frac{1}{3} - 1)}$

Step 11

Apply the distributive property.

$S_{4} = \frac{3 {(\frac{1}{3})}^{4} + 3 \cdot - 1}{3 (\frac{1}{3}) + 3 \cdot - 1}$

Step 12

Cancel the common factor of

$3$ .

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Step 12.1

Cancel the common factor.

$S_{4} = \frac{3 {(\frac{1}{3})}^{4} + 3 \cdot - 1}{3 (\frac{1}{3}) + 3 \cdot - 1}$

Step 12.2

Rewrite the expression.

$S_{4} = \frac{3 {(\frac{1}{3})}^{4} + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13

Simplify the numerator.

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Step 13.1

Apply the product rule to

$\frac{1}{3}$ .

$S_{4} = \frac{3 (\frac{1^{4}}{3^{4}}) + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13.2

Cancel the common factor of

$3$ .

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Step 13.2.1

Factor

$3$ out of

$3^{4}$ .

$S_{4} = \frac{3 (\frac{1^{4}}{3 \cdot 3^{3}}) + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13.2.2

Cancel the common factor.

$S_{4} = \frac{3 (\frac{1^{4}}{3 \cdot 3^{3}}) + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13.2.3

Rewrite the expression.

$S_{4} = \frac{\frac{1^{4}}{3^{3}} + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13.3

One to any power is one.

$S_{4} = \frac{\frac{1}{3^{3}} + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13.4

Raise

$3$ to the power of

$3$ .

$S_{4} = \frac{\frac{1}{27} + 3 \cdot - 1}{1 + 3 \cdot - 1}$

Step 13.5

Multiply

$3$ by

$- 1$ .

$S_{4} = \frac{\frac{1}{27} - 3}{1 + 3 \cdot - 1}$

Step 13.6

To write

$- 3$ as a fraction with a common denominator, multiply by

$\frac{27}{27}$ .

$S_{4} = \frac{\frac{1}{27} - 3 \cdot \frac{27}{27}}{1 + 3 \cdot - 1}$

Step 13.7

Combine

$- 3$ and

$\frac{27}{27}$ .

$S_{4} = \frac{\frac{1}{27} + \frac{- 3 \cdot 27}{27}}{1 + 3 \cdot - 1}$

Step 13.8

Combine the numerators over the common denominator.

$S_{4} = \frac{\frac{1 - 3 \cdot 27}{27}}{1 + 3 \cdot - 1}$

Step 13.9

Simplify the numerator.

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Step 13.9.1

Multiply

$- 3$ by

$27$ .

$S_{4} = \frac{\frac{1 - 81}{27}}{1 + 3 \cdot - 1}$

Step 13.9.2

Subtract

$81$ from

$1$ .

$S_{4} = \frac{\frac{- 80}{27}}{1 + 3 \cdot - 1}$

Step 13.10

Move the negative in front of the fraction.

$S_{4} = \frac{- \frac{80}{27}}{1 + 3 \cdot - 1}$

Step 14

Simplify the denominator.

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Step 14.1

Multiply

$3$ by

$- 1$ .

$S_{4} = \frac{- \frac{80}{27}}{1 - 3}$

Step 14.2

Subtract

$3$ from

$1$ .

$S_{4} = \frac{- \frac{80}{27}}{- 2}$

Step 15

Multiply the numerator by the reciprocal of the denominator.

$S_{4} = - \frac{80}{27} \cdot \frac{1}{- 2}$

Step 16

Cancel the common factor of

$2$ .

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Step 16.1

Move the leading negative in

$- \frac{80}{27}$ into the numerator.

$S_{4} = \frac{- 80}{27} \cdot \frac{1}{- 2}$

Step 16.2

Factor

$2$ out of

$- 80$ .

$S_{4} = \frac{2 (- 40)}{27} \cdot \frac{1}{- 2}$

Step 16.3

Factor

$2$ out of

$- 2$ .

$S_{4} = \frac{2 \cdot - 40}{27} \cdot \frac{1}{2 \cdot - 1}$

Step 16.4

Cancel the common factor.

$S_{4} = \frac{2 \cdot - 40}{27} \cdot \frac{1}{2 \cdot - 1}$

Step 16.5

Rewrite the expression.

$S_{4} = \frac{- 40}{27} \cdot \frac{1}{- 1}$

Step 17

Multiply

$\frac{- 40}{27}$ by

$\frac{1}{- 1}$ .

$S_{4} = \frac{- 40}{27 \cdot - 1}$

Step 18

Multiply

$27$ by

$- 1$ .

$S_{4} = \frac{- 40}{- 27}$

Step 19

Dividing two negative values results in a positive value.

$S_{4} = \frac{40}{27}$