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What is the sum of the infinite geometric series?

[tex]\[ \sum_{n=1}^{\infty}(-144)\left(\frac{1}{2}\right)^{n-1} \][/tex]

A. [tex]\(-288\)[/tex]

B. [tex]\(-216\)[/tex]

C. [tex]\(-144\)[/tex]

D. [tex]\(-72\)[/tex]


Sagot :

To find the sum of the infinite geometric series [tex]\(\sum_{n=1}^{\infty}(-144)\left(\frac{1}{2}\right)^{n-1}\)[/tex], we need to identify the first term and the common ratio of the series.

1. Identify the first term [tex]\(a\)[/tex]:
The given series can be written in a general form for a geometric series:
[tex]\[ \sum_{n=1}^{\infty} a \cdot r^{n-1} \][/tex]
Here, the first term [tex]\(a\)[/tex] is [tex]\(-144\)[/tex].

2. Identify the common ratio [tex]\(r\)[/tex]:
The common ratio [tex]\(r\)[/tex] is the factor by which each term is multiplied to get the next term. In this series, [tex]\(r = \frac{1}{2}\)[/tex].

3. Formula for the sum of an infinite geometric series:
The sum [tex]\(S\)[/tex] of an infinite geometric series [tex]\(\sum_{n=1}^{\infty} ar^{n-1}\)[/tex] is given by the formula:
[tex]\[ S = \frac{a}{1 - r} \][/tex]
This formula applies when the absolute value of the common ratio is less than 1 ([tex]\(|r| < 1\)[/tex]).

4. Substitute the values into the formula:
[tex]\[ a = -144, \quad r = \frac{1}{2} \][/tex]
Plug these into the formula:
[tex]\[ S = \frac{-144}{1 - \frac{1}{2}} \][/tex]

5. Simplify the expression:
[tex]\[ S = \frac{-144}{1 - \frac{1}{2}} = \frac{-144}{\frac{1}{2}} \][/tex]
Dividing by [tex]\(\frac{1}{2}\)[/tex] is equivalent to multiplying by 2:
[tex]\[ S = -144 \times 2 = -288 \][/tex]

Therefore, the sum of the infinite geometric series [tex]\(\sum_{n=1}^{\infty}(-144)\left(\frac{1}{2}\right)^{n-1}\)[/tex] is [tex]\(\boxed{-288}\)[/tex].