First multiply both 4 and 18:
4*18=72
According to formula:
G(a1.a2) = sqrt(a1.a2)
G(a1.a2.a3) = 1/3 root of (a1.a2.a3)
As there are only 2 numbers whose geometric mean to be calculated, we take square root of multiplication of two numbers.
Sqrt(72) = 8.4852
