1.原式=(a^4-6a^2+9)-9a^2
=(a^2-3)^2-9a^2
=(a^2-3a-3)(a^2+3a-3)
2.原式=(x^3+7x^2)-(7x^2+49x)+(x+7)
=(x+7)(x^2-7x+1)
3.原式=(2x+x^2+1)(2x-x^2-1)
=-(x+1)^2(x-1)^2
4原式=(5x^2-13y^2+4x^2-12y^2)(5x^2-13y^2-4x^2+12y^2)
=(3x+5y)(3x-5y)(x+y)(x-y)