how do I do geometric mean?

For n values, we multiply them together and then take the  nth root of that result

Ex:    geometric mean of  18  and 8

Multiply together  =  144

Since we have 2 values, we take the square root

sqrt (144)  =12    =   geometric mean of 18 and  8

One more :

Geometric mean of   2, 32, 64

Multiply them together  = 4096

Since we have three values, take the  cube root

cube root (4096)  =   16   = geometric mean of  2, 32 and 64