High School

Identify the 16th term of a geometric sequence where [tex]$a_1 = 4$[/tex] and [tex]$a_8 = -8,748$[/tex].

A. [tex]$-172,186,884$[/tex]
B. [tex]$-57,395,628$[/tex]
C. [tex]$57,395,628$[/tex]
D. [tex]$172,186,884$[/tex]

Answer :

To find the 16th term of a geometric sequence, we start by using the formula for the nth term of a geometric sequence:

[tex]\[ a_n = a_1 \cdot r^{(n-1)} \][/tex]

where [tex]\( a_n \)[/tex] is the nth term, [tex]\( a_1 \)[/tex] is the first term, and [tex]\( r \)[/tex] is the common ratio.

We are given the first term ([tex]\( a_1 = 4 \)[/tex]) and the 8th term ([tex]\( a_8 = -8,748 \)[/tex]). We need to find the common ratio [tex]\( r \)[/tex].

### Step 1: Find the Common Ratio

We use the formula for the 8th term:

[tex]\[ a_8 = a_1 \cdot r^{(8-1)} \][/tex]

Substitute the values:

[tex]\[ -8,748 = 4 \cdot r^7 \][/tex]

To find [tex]\( r \)[/tex], solve for [tex]\( r^7 \)[/tex]:

[tex]\[ r^7 = \frac{-8,748}{4} \][/tex]
[tex]\[ r^7 = -2,187 \][/tex]

Now find the 7th root of -2,187 to find [tex]\( r \)[/tex].

### Step 2: Find the 16th Term

Once we have the common ratio, we can find the 16th term ([tex]\( a_{16} \)[/tex]) using:

[tex]\[ a_{16} = a_1 \cdot r^{(16-1)} \][/tex]
[tex]\[ a_{16} = 4 \cdot r^{15} \][/tex]

After calculating, we find that the 16th term of the sequence is approximately [tex]\( 57,395,628 \)[/tex].

Therefore, the 16th term of the geometric sequence is [tex]\(\boxed{57,395,628}\)[/tex].