十字相乘法,(x+y)^2-3(x+y)-28

问题描述:

十字相乘法,(x+y)^2-3(x+y)-28
a^2-4ab-5b^2
x^2+5xy-6y^2
x^3-6x^2-72x
x^2y^2-7xy-18

,(x+y)^2-3(x+y)-28=(x+y-7)(x+y+4)
a^2-4ab-5b^2=(a-5b)(a+b)
x^2+5xy-6y^2=(x+6y)(x-y)
x^3-6x^2-72x=x(x-12)(x+6)
x^2y^2-7xy-18 =(xy-9)(xy+2)