9. Geometric; $a_1=3,r=2$

11. Not geometric

13. Geometric; $a_1=1,r=-3$

15. Geometric; $a_1=7,r=-1$

17. Geometric; $a_1=9,r={\displaystyle \frac{1}{3}}$

19. Geometric; $a_1=-{\displaystyle \frac{1}{2}},r=-{\displaystyle \frac{1}{2}}$

21. Geometric; $a_1=1,r=2$

23. Not geometric

25. Geometric; $a_1={\displaystyle \frac{1}{3}},r={\displaystyle \frac{1}{3}}$

27. Geometric; $a_1=\sqrt{5},r=\sqrt{5}$

29. $a_1=2,r=5,a_n=2{(5)}^{n-1}$

31. $a_1=5,r={\displaystyle \frac{2}{3}},a_n=5{\left({\displaystyle \frac{2}{3}}\right)}^{n-1}$

33. $a_1=0.2,r=-3,$ $a_n=0.2{(-3)}^{n-1}$

35. $a_1={\pi }^4,r={\pi }^2,a_n={\pi }^{2n+2}$

37. $a_n=3\cdot 2^n$

39. $a_n=108\cdot {\left({\displaystyle \frac{1}{3}}\right)}^n$

41. $a_n=-{\displaystyle \frac{1}{5}}\cdot 4^n$

43. $a_7=320$

45. $a_{10}=-1536$

47. $a_6={\displaystyle \frac{243}{16}}$

49. $a_9=-{\displaystyle \frac{390,625}{256}}$

51. $a_{20}=-{\displaystyle \frac{125}{131,072}}$

53. 11

55. 10

57. 9

59. $S_{10}={\displaystyle \frac{1\text{,}220\text{,}703}{5}}$

61. $S_{12}=-{\displaystyle \frac{40,690,104}{25}}$

63. $S_8={\displaystyle \frac{109,225}{16,384}}$

65. ${\displaystyle \frac{31}{16}}$

67. ${\displaystyle \frac{6305}{729}}$

69. 255

71. ${\displaystyle \frac{3(5^{20}-2^{20})}{35{(2)}^{19}}}$

73. 0.397

75. 9841.333

77. 2.250

79. ${\displaystyle \frac{1}{2}}$

81. $-{\displaystyle \frac{1}{3}}$

83. ${\displaystyle \frac{32}{5}}$

85. ${\displaystyle \frac{15}{2}}$

87. ${\displaystyle \frac{4}{5}}$