since a,b,c are in GP, their common ratio is .  Therefore 



Raise all three parts to the  power



Multiply exponents:



(1)   

From the first two expressions of (1)



Raise both sides to the z power

(2)  

From the first and third expressions of (1)



Raise both sides to the y power:

 

Multiply both sides by :

(3)  

Square both sides of (2)



Substitute  for  in (3)   

(3)  

Equate exponents of a in (3)









Therefore



Edwin