-4 is also a geometric mean
if we consider a(first element in series)=1 and r(common ratio)=-4 then the series would be
1,-4,16,-64,256,…..,a(-4)^(n-1 )
where n is nth term
from this -4 would be the geometric mean if we consider -4 as common ratio .
If we consider 4 as common ratio then geometric mean should be 4
so you should mention whether common ratio >0 or not (r>0 or not) .
[without ‘r’ value you can’t solve the question but in general most of the teachers will consider r>0.]
So, -4 won’t be geometric mean of 1 & 16
