Part 1)
3,6,12,24,48 ... Is geometric ,with ratio= 6/3=2
-4,+1,+6,+11... is arithmetic , ratio 1--4= 5
100,50,25,12,5... Is geometric , decreasing ratio= 50/100= 1/2
162, 108, 72, 48... Is Neither
40.5, 13.5, 4.5, 1.5... is geometric, decreasing ratio= 1/3
3,8,13,18,23 ... Is arithmetic, increasing ratio = 5
Part 2)5
an = -2(-5)^n
First five terms are
a1= -2
a2= 10
a3 = -50
a4 = +250
a5 = -1250