Question

Find the first term and common ratio for the geometric sequence a2 =2.5,a4=62.5

Answer 1

a1 = 0.5; common ratio = 5

Explanation:

Let the common ratio = r

a2 = 2.5

a3 = 2.5r

a4 = 2.5r * r = 2.5r^2

We are told a4 = 62.5, so

2.5r^2 = 62.5

Divide both sides by 2.5

r^2 = 25

r = 5

a2 = a1 * r

a1 = a2/r = 2.5/5 = 0.5

Answer: a1 = 0.5; common ratio = 5