Answer:
((3 x^2 - 1) (3 x^2 + 1) (9 x^4 + 1))/81
Explanation:
Factor the following: 
x^8 - 1/81 
 
Put each term in x^8 - 1/81 over the common denominator 81: x^8 - 1/81 = (81 x^8)/81 - 1/81: 
(81 x^8)/81 - 1/81 
 
(81 x^8)/81 - 1/81 = (81 x^8 - 1)/81: 
(81 x^8 - 1)/81 
 
81 x^8 - 1 = (9 x^4)^2 - 1^2: 
((9 x^4)^2 - 1^2)/81 
 
Factor the difference of two squares. (9 x^4)^2 - 1^2 = (9 x^4 - 1) (9 x^4 + 1): 
((9 x^4 - 1) (9 x^4 + 1))/81 
 
9 x^4 - 1 = (3 x^2)^2 - 1^2: 
((3 x^2)^2 - 1^2 (9 x^4 + 1))/81 
 
Factor the difference of two squares. (3 x^2)^2 - 1^2 = (3 x^2 - 1) (3 x^2 + 1): 
Answer: ((3 x^2 - 1) (3 x^2 + 1) (9 x^4 + 1))/81