# Find the 13th term of the geometric sequence 7, 21, 63, …

Find the 13th term of the geometric sequence 7, 21, 63, ...

## This Post Has 10 Comments

1. babycakesmani says:

a=4

r = \frac{-16}{4}r=4−16

r = -4r=−4

a_{13} = 4×(-4)^{(13-1)}a13=4×(−4)(13−1)

a_{13} = 4×(-4)^{12}a13=4×(−4)12

a_{13} = 4×16777216a13=4×16777216

a_{13} = 67108864a13=67108864

2. danielzgame says:

A geometric sequence is a sequence with a common ratio multiplied by previous term to give the next. Lets call x the common ratio, so we know that 4 was multiplied by x to give the second term, and so forth until it was multiplied again and again by x until the 16384 was reached which happens to be the 13th term, that means that 4x was multiplied by x 12 more times to give 16384, that is:
4x^12 = 16384
x^12 = 4096
x = 4096^-12
x = 4.48 x 10^-44
that is the common ratio

3. ashleyzamarripa08 says:

3,720,087 sorry if I’m wrong

4. lexibyrd120 says:

A

Step-by-step explanation:

You plug in 13 to n-1. to get -2^12. This equates to 4096, and you multiply that by 3

5. NetherisIsTheQueen says:

6377292

Step-by-step explanation:

it keeps on multiplying by 3.

6. nickocasamplonp6mlob says:

$\large \boxed{2}$

Step-by-step explanation:

The formula for the nth term of a geometric sequence is

aₙ = a₁rⁿ⁻¹

In your geometric sequence, a₁ = 4 and a₁₃ = 16 384.

$\begin{array}{rcl}16384 & = & 4r^{(13 - 1)}}\\16384 & = & 4r^{12}\\4096 & = & r^{12}\\3.6124 & = & 12 \log r\\0.30102 & = & \log r\\r & = & 10^{0.30102}\\ & = & \mathbf{2}\\\end{array}\\\text{The common ratio is \large \boxed{\mathbf{2}}}$

Check:

$\begin{array}{rcl}16384 & = & 4(2)^{12}\\16384 & = & 4(4096)\\16384 & = & 16384\\\end{array}$

It checks.

7. greyxxamber says:

-875

Step-by-step explanation:

the pattern is to multiply the last number by -5.

8. maxdmontero says:

20, 480

Step-by-step explanation:

The sequence is just x multiplied by 2

If you follow the sequence, on the 13th term, the number is 20,480.

9. caplode5112 says:

4096

Step-by-step explanation:

The n th term of a geometric sequence is

$a_{n}$ = a$(r)^{n-1}$

where a is the first term and r the common ratio

Here a = 1 and r = 2 ÷ 1 = 4 ÷ 2 = 2, thus

$a_{13}$ = 1 × $2^{12}$ = 4096

10. xdoran says:

1708984375

Step-by-step explanation:

In order to get from 7 to -35 you need to multiply by -5

so that is what it is increasing by... a multiple of -5

7, -35 , 175, -875, 4375, -21875, 109375, -546875, 2734375, -13671875, 68359375, -341796875, 1708984375