In a geometric sequence, a=72 and a13= 1/2 use the geometric mean to find the value of a12

If a=72

a13=1/2
72*13 =1/2
936 =1/2

Than

a12=72*12=864
a12=a13-a1

a12= 12/26 or 6/13

Why?

Because if 13 times "a" is 1/2 ( which means half of a number)

The full number is twice as much:
a13 times 2 = a26 or 1/1

Than a12 is the 12 part of the whole 26 which means a12 = 12/26

If we simplify 12 dived by 26 is:

6/13

a12=6/13