(7a-8b-4x)(7a-8b+4x)
(x-4)(x+y)(x-y)
(x^2+y^2-xy)(x^2+y^2+xy)
(2a-b)3b
-(x+y)(x+9y)
-(5m+n)(m+3n)
都是平方差公式 a^2-b^2=(a+b)(a-b)