ax²－ay²
=a(x²-y²)
=a(x+y)(x-y)
a（x-y）-b（y-x）+c（x-y）
=a(x-y)+b(x-y)+c(x-y)
=(x-y)(a+b+c)第一题在写过一遍好理解的来、谢谢