（1） x^2-9-2xy+y^2
=（x+y)^2-9
=（x+y+3)(x+y-3)
(2) a^2-9ab+14b^2
=（a-2b)(a-7b)
(3)4a^2-b^2-4a+1
=（2a-1)^2-b^2
=（2a+b-1)(2a-b-1)
(4)5(x^2+1)^2-20x^2
=5[(x^2+1)^2-4x^2]
=5(x^2+1+2x)(x^2+1-2x)
=5(x+1)^2(x-1)^2