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A geometric sequence has second term 12 and fifth term 324. A) find common ratio b) calculate 10th term c) the kth term is the first term th
Question
A geometric sequence has second term 12 and fifth term 324. A) find common ratio b) calculate 10th term c) the kth term is the first term that is greater than 2000. Find the value of K
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Mathematics
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2021-08-13T12:57:13+00:00
2021-08-13T12:57:13+00:00 1 Answers
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Answers ( )
Answer:
a) find common ratio = 3
b) calculate 10th term = 78732
c) k = 7
Step-by-step explanation:
A geometric sequence has second term 12 and fifth term 324.
a) find common ratio
Common ratio = r
Hence
We solve for this below:
r = cube root(fifth term/second term)
r = cube root (324/12)
r = cube root (27)
r = 3
Therefore, the common ratio = 3
b) calculate 10th term
The formula for a geometric sequence is given as:
an = ar^n-1
Where
a = First term
r = Common ratio = 3
n = Nth term = 10
Step 1
We have to find the first term
Common ratio = Second term/First term
Common ratio = 3
Second term = 12
Hence:
3 = 12/x
Cross Multiply
3x = 12
x = 12/3
x = 4
Hence , First term = 4
Step 2
We find the 10th term
an = ar^n-1
a10 = 4 × 3^10 – 1
a10 = 4 × 3⁹
a10 = 4 × 19683
a10 = 78732
Therefore, the 10th term = 78732
c) the kth term is the first term that is greater than 2000. Find the value of K
For kth term,
ak = ar^k-1 >2000
a = 4, r = 3
Hence
4 × 3^k-1 > 2000
We divide both sides by 4
4 × 3^k-1/4 > 2000/4
3^k-1 > 500
We take the logarithm of both sides
log 3^k-1 > log 500
k-1 log 3 > log 500
Divide both sides by log 3
k-1 log 3/log 3 > log 500/log 3
k – 1 > 5.6567800693
k > 5.6567800693 + 1
k > 6.6567800693
k = 7
Therefore, k = 7