geometric is when each term increased by the same ratio as the previous one
example
if there are terme, x,y,z in a seqence
the sequence is geometric if
y/x=z/y
so
first one
-4/2=-6/4, -2=-1.5, no
2nd
-8/16=4/-8, -1/2=-1/2, yes
3rd one
-18/-15=-21.6/-18, 1.2=1.2, yes
4th one
10.5/4=17/10.5, 2.625=1.619, no
5th one
125/625=25/125, 1/5=1/5, yes
the ones that are geometric are
16, –8, 4, –2, 1
–15, –18, –21.6, –25.92, –31.104, …
625, 125, 25, 5, 1, …