Answer:
a) 
b) 
c) 
Step-by-step explanation:
a) Geometric sequence with first term 5 and common ratio 2, where the nth term can be calculated via:

The first five terms are: 
b) Geometric sequence with first term 100 and common ratio 1/2, where the nth term can be calculated via:

The first five terms are: 
c) Geometric sequence with first term 160 and common ratio -1/2, where the nth term can be calculated via:

The first five terms are: 