The first figure of the Sierpinski triangle has one shaded triangle. The second figure of the Sierpinski triangle has three shaded triangles. The third figure of the Sierpinski triangle has nine shaded triangles. Which summation represents the total number of shaded triangles in the first 15 figures?

A. [tex]\sum_{n=1}^{15} 1(3)^{n-1}[/tex]

B. [tex]\sum_{n=1}^{15} 3(1)^{n-1}[/tex]

C. [tex]\sum_{n=1}^{15} 1\left(\frac{1}{3}\right)^{n-1}[/tex]

D. [tex]\sum_{n=1}^{15} \frac{1}{3}(1)^{n-1}[/tex]



Answer :

To determine which summation represents the total number of shaded triangles in the first 15 figures of the Sierpinski triangle, let's analyze each of the provided summations one by one:

1. [tex]\(\sum_{n=1}^{15} 1(3)^{n-1}\)[/tex]

- This summation considers the expression [tex]\( 1 (3)^{n-1} \)[/tex].
- When [tex]\( n = 1 \)[/tex]: [tex]\( 1 (3)^{1-1} = 1 (3)^0 = 1 \)[/tex]
- When [tex]\( n = 2 \)[/tex]: [tex]\( 1 (3)^{2-1} = 1 (3)^1 = 3 \)[/tex]
- When [tex]\( n = 3 \)[/tex]: [tex]\( 1 (3)^{3-1} = 1 (3)^2 = 9 \)[/tex]
- And so on...
- This pattern matches the one given (1, 3, 9...).

Hence, the total number of shaded triangles in the first 15 figures would be:

[tex]\[ 1 (3)^{15-1} = 4782969 \][/tex]

2. [tex]\(\sum_{n=1}^{15} 3(1)^{n-1}\)[/tex]

- This summation considers the expression [tex]\( 3 (1)^{n-1} \)[/tex].
- When [tex]\( n = 1 \)[/tex]: [tex]\( 3 (1)^0 = 3 \)[/tex]
- When [tex]\( n = 2 \)[/tex]: [tex]\( 3 (1)^1 = 3 \)[/tex]
- When [tex]\( n = 3 \)[/tex]: [tex]\( 3 (1)^2 = 3 \)[/tex]
- And so on...
- The result for each term in this series is always [tex]\( 3 \)[/tex].

Hence, the total for the first 15 figures is [tex]\( 3 \times 15 = 45 \)[/tex]. However, considering only the last term as given:

[tex]\[ 3 (1)^{15-1} = 3 \][/tex]

3. [tex]\(\sum_{n=1}^{15} 1\left(\frac{1}{3}\right)^{n-1}\)[/tex]

- This summation considers the expression [tex]\( 1 \left( \frac{1}{3} \right)^{n-1} \)[/tex].
- When [tex]\( n = 1 \)[/tex]: [tex]\( 1 \left( \frac{1}{3} \right)^0 = 1 \)[/tex]
- When [tex]\( n = 2 \)[/tex]: [tex]\( 1 \left( \frac{1}{3} \right)^1 = \frac{1}{3} \)[/tex]
- When [tex]\( n = 3 \)[/tex]: [tex]\( 1 \left( \frac{1}{3} \right)^2 = \left( \frac{1}{3} \right)^2 = \frac{1}{9} \)[/tex]
- The terms get smaller (fractions less than 1).

Considering the last term for [tex]\( n = 15 \)[/tex]:

[tex]\[ 1\left(\frac{1}{3}\right)^{15-1} = 2.090751581287688 \times 10^{-7} \][/tex]

4. [tex]\(\sum_{n=1}^{15} \frac{1}{3}(1)^{n-1}\)[/tex]

- This summation considers the expression [tex]\( \frac{1}{3} (1)^{n-1} \)[/tex].
- When [tex]\( n = 1 \)[/tex]: [tex]\( \frac{1}{3} (1)^0 = \frac{1}{3} \)[/tex]
- When [tex]\( n = 2 \)[/tex]: [tex]\( \frac{1}{3} (1)^1 = \frac{1}{3} \)[/tex]
- When [tex]\( n = 3 \)[/tex]: [tex]\( \frac{1}{3} (1)^2 = \frac{1}{3} \)[/tex]
- The value remains [tex]\( \frac{1}{3} \)[/tex] for each term.

Hence, for the last term at [tex]\( n = 15 \)[/tex]:

[tex]\[ \frac{1}{3} (1)^{15-1} = \frac{1}{3} = 0.3333333333333333 \][/tex]

From analyzing all the given options, it is evident that the correct summation representing the total number of shaded triangles in the first 15 figures is:

[tex]\[ \sum_{n=1}^{15} 1(3)^{n-1} \][/tex]

Therefore, we select:
\[
\sum_{n=1}^{15} 1(3)^{n-1}.