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