Question

Determine if the sequence is geometric . If it is , find the common ratio , the explicit formula and the recursive formula .

1. -40 , 20 , -100 , 500


2. 4 , 20 , 100 , 500


3. 4 , 24 , 144, 864


5. 3 , -12 , 48 , -192

Answer 1

Explanation:

if it is a geometric sequence, then

sn = sn-1 × r

with r being the common ratio.

1.

s2 = s1 × r

20 = -40 × r

r = 20/-40 = -1/2

but the same r has to apply to all other terms, if it is geometric.

s3 = s2 × r

-100 = 20 × -1/2 = -10

this is wrong, so, it is not geometric.

2.

s2 = s1 × r

20 = 4 × r

r = 20/4 = 5

but the same r has to apply to all other terms, if it is geometric.

s3 = s2 × r

100 = 20 × 5 = 100

s4 = s3 × r

500 = 100 × 5 = 500

this is all correct, so, it is geometric.

sn = sn-1 × 5

sn = s1 × 5^(n-1) = 4 × 5^(n-1)

3.

s2 = s1 × r

24 = 4 × r

r = 24/4 = 6

but the same r has to apply to all other terms, if it is geometric.

s3 = s2 × r

144 = 24 × 6 = 144

s4 = s3 × r

864 = 144 × 6 = 864

this is all correct, so, it is geometric.

sn = sn-1 × 6

sn = s1 × 6^(n-1) = 4 × 6^(n-1)

5.

s2 = s1 × r

-12 = 3 × r

r = -12/3 = -4

but the same r has to apply to all other terms, if it is geometric.

s3 = s2 × r

48 = -12 × -4 = 48

s4 = s3 × r

-192 = 48 × -4 = -192

this is all correct, so, it is geometric.

sn = sn-1 × -4

sn = s1 × (-4)^(n-1) = 3 × (-4)^(n-1)