1. Review
Find the explicit formula for each arithmetic sequence.
1) a1 = 1 and a5 = 13
2) a3 = 13 and a7 = 29

2. Answers:
1) an = 3n - 2
2) an = 4n + 1

3. Key Concepts for Sect. 9.4
*A sequence is geometric if the ratios of consecutive terms are the same.
geometric: 2, 4, 8, 16, 32,...
1, 3, 9, 27, 81,...
not geometric: 5, 10, 15, 20, 25,...
1, 4, 9, 16, 25,...

4. Key Concepts for Sect. 9.4
*To find the explicit rule of a geometric sequence, use the formula below:
an = a1 rn - 1

5. Guided Practice for Sect. 9.4
Ex 1: Find the explicit rule and the recursive rule for the geometric sequence.
a) 5, -15, 45, -135,...

6. Ex 1:
b) 81, 27, 9, 3,...

7. Guided Practice for Sect. 9.4
Ex 2: Find the explicit rule for the geometric sequence.
a) a1 = 5, a3 = 45/4

8. Ex 2:
b) a3 = 16/3, a5 = 64/27