计算(x-8y)(x-y)-(x+y)的平凡

问题描述:

计算(x-8y)(x-y)-(x+y)的平凡

(x-8y)(x-y)-(x+y)^2
= x^2 - xy - 8xy + 8(y^2) - (x + y)(x + y)
= x^2 - 9xy + 8(y^2)- (x^2 + xy + xy + y^2 )
= x^2 - 9xy + 8(y^2)- (x^2 + 2xy + y^2)
= x^2 - 9xy + 8(y^2)- x^2 - 2xy - y^2
= -11xy + 7(y^2)
注:x^2就是x的平方