x²-2x-15 先说这一道
=x²-2x+1-1-15
=(x-1)²-16
=(x-1)²-4²
=(x-1+4)(x-1-4)
=(x+3)(x-5)
(x+p)²-(x+q)² 再是这道题
=(x+p+x+q)(x+p-x-q)
=(2x+p+q)(p-q)
3ax+4by+4ay+3bx 最后这道有好几种解法:
普通法:=3ax+3bx4ay+4by
=3x(a+b)+4y(a+b)
=(a+b)(3x+4y)
十字相乘法:3ax+4by+4ay+3bx
=3ax+4ay+3bx+4by
a b
3x 4y
=(a+b)(3x+4y)