Evaluate sigma-summation Underscript lower i equals 1 overscript infinity EndScripts 26 left parenthesis start fraction 1 over 5 end fraction right parenthesis superscript lower i baseline.

(1 point)
Responses

26
26

6.5
6.5

32.5
32.5

130

1 answer

To evaluate the summation

\[ \sum_{i=1}^{\infty} 26 \left(\frac{1}{5}\right)^i, \]

we recognize this as a geometric series of the form

\[ \sum_{i=0}^{\infty} ar^i, \]

where \(a\) is the first term and \(r\) is the common ratio. In our case:

  • The first term \(a\) for \(i = 1\) is \(26 \frac{1}{5} = \frac{26}{5}\),
  • The common ratio \(r = \frac{1}{5}\).

The formula for the sum of an infinite geometric series is given by:

\[ S = \frac{a}{1 - r}, \]

provided that \(|r| < 1\). Here, \( |r| = \frac{1}{5} < 1 \), so we can use the formula:

Calculating the sum:

\[ S = \frac{\frac{26}{5}}{1 - \frac{1}{5}} = \frac{\frac{26}{5}}{\frac{4}{5}} = \frac{26}{5} \cdot \frac{5}{4} = \frac{26}{4} = 6.5. \]

Thus, the value of the summation is

\[ \sum_{i=1}^{\infty} 26 \left(\frac{1}{5}\right)^i = 6.5. \]

The correct response is:

6.5