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