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Step-by-step 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)