Find the nth term of the following geometric sequence.
a. 1, 6, -36, 216,... (10th term)
b. -2, -4, -8, -16,... (11th term)
c. -3, -15, -75, -375,... (12th term)
d. 1, -5, 25, -125,... (13th term)
e. 2, 1/2, 1/8,... (14th term)