Answer: 12
======================================================
Work Shown:
z = geometric mean of x and y
z = sqrt(x*y)
z = sqrt(8*18)
z = sqrt(144)
z = 12
The geometric mean of 8 and 18 is 12.
The geometric mean is similar to the arithmetic mean; however instead of add up the values and divide by n, this time we're multiplying the values and then applying the square root. This only applies if we are finding the geometric mean of 2 values. If you have more values, then you'll need to take higher order roots.
Eg: if you had 3 values, then you would need to take a cube root. Four values needs a fourth root, and so on.