1) 20a^2-45b^2
5(4a^2-9b^2)
5(2a-3b)(2a+3b)
2) 128a^3b-2ab^3
2ab(64a^2-b^2)
2ab(8a-b)(8a+b)
3) x^4-y^4
(x^2-y^2)(x^2+y^2)
(x-y)(x+y)(x^2+y^2)
4) x^8-y^8
(x^4-y^4)(x^2+y^2)
(x^2-y^2)(x^2+y^2)(x^2+y^2)
(x-y)(x+y)(x^2+y^2)(x^2+y^2)
5) a^4-1
(a^2-1)(a^2+1)
(a-1)(a+1)(a^2+1)
6) 1-81y^4
(1-9y^2)(1+9y^2)
(1-3y)(1+3y)(1+9y^2)