Question

先化简，再求值：（x-y)(x+y)-(x+y)^2+2y(y-x)，其中x=1，y=3。

拜托了。

Answer 1

解：
原式
=(x-y)(x+y)-(x+y)^2+2y(y-x)
=(x+y)(x-y-x-y)-2y(x-y)
=-2y(x+y)-2y(x-y)
=-2y(x+y+x-y)
=-2y×(-2x)
=4xy
将x=1，y=3代入，有：
原式=4xy=4×1×3=12。

Answer 2

（x-y)(x+y)-(x+y)^2+2y(y-x)
=(x-y)(x+y-2y)-(x+y)^2
=(x-y)^2-(x+y)^2
=(x-y+x+y)(x-y-x-y)
=2x*(-2y)
=-4xy
=-4*1*3=-12