How to find the first term of a geometric sequence given two terms

Im assuming you are given terms 2 and 3  and you want the first term.

Divide the  third term by the second term  The result will give you the common ratio r.

first term is a1 and second is a1 r

a1 = a1 r /   r

so you need to divide the second term by common ratio r. This will give you the first term

The answer is "a"radius = 12.7 unitsdiameter = radius × 212.7 units × 2 = 25.4 units