a)
25a^2-16b^2=(5a+4b)(5a-4b)
16x^2-36y^2=(4x^2+6y)(4x^2-6y)
8a^6-25=(\sqrt8a^3+5)(\sqrt8a^3-5)=(2\sqrt2a^3+5)(2\sqrt2a^3-5)
b)
(x-y)^2-9=(x-y+3)(x-y-3)
121x^2y^2-(2a+3b)^2=[11xy+(2a+3b)][11xy-(2a+3b)]
\frac{(x+y)^2}{16}-\frac{(x-y)^2}{81}=(\frac{(x+y}{4})^2-(\frac{x-y}{9})^2=(\frac{x+y}{4}+\frac{x-y}{9})(\frac{x+y}{4}-\frac{x-y}{9})