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What is the third term in the geometric sequence [tex]\( f(n)=2(0.8)^{n-1} \)[/tex]?

A. 4.8
B. 2.56
C. 1.28
D. 1.024


Sagot :

To find the third term in the geometric sequence [tex]\( f(n) = 2 \cdot (0.8)^{n-1} \)[/tex], follow these steps:

1. Identify the general form of the geometric sequence:
[tex]\[ f(n) = a \cdot r^{n-1} \][/tex]
where [tex]\( a \)[/tex] is the first term, [tex]\( r \)[/tex] is the common ratio, and [tex]\( n \)[/tex] is the term number.

2. Given the sequence, [tex]\( a = 2 \)[/tex] and [tex]\( r = 0.8 \)[/tex].

3. We are asked to find the third term ([tex]\( n = 3 \)[/tex]). Substitute [tex]\( n = 3 \)[/tex] into the general form of the sequence:
[tex]\[ f(3) = 2 \cdot (0.8)^{3-1} \][/tex]

4. Simplify the exponent [tex]\( 3-1 \)[/tex]:
[tex]\[ f(3) = 2 \cdot (0.8)^2 \][/tex]

5. Calculate [tex]\( (0.8)^2 \)[/tex]:
[tex]\[ (0.8)^2 = 0.64 \][/tex]

6. Multiply [tex]\( 2 \)[/tex] by [tex]\( 0.64 \)[/tex]:
[tex]\[ f(3) = 2 \cdot 0.64 = 1.28 \][/tex]

The third term in the geometric sequence [tex]\( f(n) = 2 \cdot (0.8)^{n-1} \)[/tex] is [tex]\( 1.28 \)[/tex].

Therefore, the answer is [tex]\(\boxed{1.28}\)[/tex].
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