This is a geometric sequence because there is a common ration. Which means that each term is a constant multiple of the previous term...in this case:
r=(-1/4)/(1/2)=(1/8)/(-1/4)=-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(n)=(1/2)(-1/2)^(n-1)
