If where
are in A.P. and
and a, b, c re in A.P
x = 1 + a + a2 +.........
y = 1 + b + b2 +........
z = 1 + c + c2 +.........
Now, since a, b, c are in A.P
So -a, -b, -c are in A.P
So 1-a, 1-b, 1-c, are in A.P
So in A.P
So x, y, z are in H.P