4a^2-b^2
=（2a+b)(2a-b)
-25x^2+4
=(2+5x)(2-5x)
a^4-81
=(a^2+9)(a^2-9)
=(a^2+9)(a+3)(a-3)
4(x+1)^2-9
=(2x+2+3)(2x+2-3)
=(2x+5)(2x-1)
(3x+2y)^2-(2x+3y)^2
=(3x+2y+2x+3y)(3x+2y-2x-3y)
=(5x+5y)(x-y)
=5(x+y)(x-y)
x^2(a-1)+4(1-a)
=x^2(a-1)-4(a-1)
=(a-1)(x^2-4)
=(a-1)(x+2)(x-2)