11.3 – Geometric Sequences
What is a Geometric Sequence? The ratio of a term to it’s previous term is constant. This means you multiply by the same number to get each term. This number that you multiply by is called the common ratio (r).
Ex: Determine if the sequence is geometric Ex: Determine if the sequence is geometric. If so, identify the common ratio 1, -6, 36, -216 yes. Common ratio=-6 2, 4, 6, 8 no. No common ratio
Important Formula for Geometric Sequence: Explicit Formula an = a1 * r n-1 Where: an is the nth term in the sequence a1 is the first term n is the number of the term r is the common ratio
Ex: Write the explicit formula for each sequence First term: a1 = 7 Common ratio = 1/3 Explicit: an = a1 * r n-1 a1 = 7(1/3) (1-1) = 7 a2 = 7(1/3) (2-1) = 7/3 a3 = 7(1/3) (3-1) = 7/9 a4 = 7(1/3) (4-1) = 7/27 a5 = 7(1/3) (5-1) = 7/81 Now find the first five terms:
Explicit Arithmetic Sequence Problem Find the 19th term in the sequence of 11,33,99,297 . . . an = a1 * r n-1 Start with the explicit sequence formula Find the common ratio between the values. Common ratio = 3 a19 = 11 (3) (19-1) Plug in known values a19 = 11(3)18 =4,261,626,379 Simplify
Find the 10th term in the sequence of 1, -6, 36, -216 . . . Let’s try one Find the 10th term in the sequence of 1, -6, 36, -216 . . . an = a1 * r n-1 Start with the explicit sequence formula Find the common ratio between the values. Common ratio = -6 a10 = 1 (-6) (10-1) Plug in known values a10 = 1(-6)9 = -10,077,696 Simplify