（1）x2-9-2xy+y2
=(x-y)²-9
=(x-y+3)(x-y-3)
（2）x2+2x+1-y2
=(x+1)²-y²
=(x+1+y)(x+1-y)
（3）4a2-b2-4a+1
=(2a-1)²-b²
=(2a-1+b)(2a-1-b)
（4）9x2-1-6xy+y2
=(3x-y)²-1
=(3x-y+1)(3x-y-1)