Which of the following sequences are geometric sequences?
check all that apply.
a. 10, 13, 16.9, 21.97, 28.561, 37.1293,
b. 10, 12, 13.1, 13.31, 15.641,
c. 15, 30, 60, 120, 240,
d. 0.1, 0.01, 0.001, 0.0001, 0.1,
e. 5, 125, 225, 1,625, 3,125,

Geometric sequence: a ( n ) = a ( n - 1 ) · r
A ) 13 = 10 · r,  r = 1.3
16.9 = 13 · 1.3,  21.97 = 16.9 · 1.3,  28.561 = 21.97 · 1.3,  
37.1293 = 28.561 · 1.3 
C ) 30 = 15 · r,  r = 2, 60 = 30 · 2,  120 = 60 · 2,  240 = 120 · 2 
D ) 0.01 = 0.1 · r,   r = 0.1
0.001 = 0.01 · 0.1 ,   0.0001 = 0.001 · 0.01,  0.00001 = 0.0001 · 0.1
Geometric sequence are A ), C ) and D ).