9x^2-16y^2
=(3x+4y)(3x-4y)
25x^2y-10xy+y
=(5x-y)^2
81x^4-16y^4
=(9x^2+4y^2)(9x^2-4y^2)
=(9x^2+4y^2)(3x+2y)(3x-2y)