Question

Find the formula for the following sequence

Geometric : a7=9 and a12=2187

Answer 1

Explanation:

a geometric sequence is s sequence, where every new element is generated by multiplying the previous element by the same number.

a n = a1 × number^(n-1)

so, a12/a7 = number^5

=> 2187/9 = 243 = number^5

=>

`number = \sqrt[5]{243}`

=> number = 3

so, what is a1 ?

it has to satisfy e.g. a7.

9 = a1 × 3^6

9 = 3^2

=> 3^2 = a1 × 3^6

=> 1 = a1 × 3^4

=> a1 = 1/(3^4) = 1/81

therefore, the whole formula is

a n = 1/81 × 3^(n-1 )