That top one is a geometric sequence because it has a common ratio. The common ratio is just a constant multiple that exists between any term and the previous one. In this case it is:
-44/88=22/-44=r=-1/2
Any geometric sequence can be expressed as:
a(n)=ar^(n-1), a=initial term, r=common ratio, n=term number, in this case:
a=88 and d=-1/2 so
a(n)=88(-1/2)^(n-1) so the fourth and fifth terms are:
a(4)=88(-1/2)^(3)=-11, a(5)=88(-1/2)^4=5.5
.....
Again, this is a geometric sequence because there is a common ratio, r of:
16.8/42=6.72/16.8=r=0.4 or if you prefer 4/10, and a=42 so the rule is:
a(n)=42(4/10)^(n-1) so the fourth term is:
a(4)=42(4/10)^3=2.688
