Question

先化简，再求值，（x²-y²）/（x+y）-2x-2y，其中x=3，y=1/3

Answer 1

，（x²-y²）/（x+y）-2x-2y
=(x+y)(x-y)/(x+y)-2x-2y
=x-y-2x-2y
=-x-3y
把x=3,y=1/3代入得
原式=-3-3*（1/3）=-4

Answer 2

（x²-y²）/（x+y）-2x-2y
=x-y-2x-2y
=-x-3y
其中x=3，y=1/3
-3-1=-4

Answer 3

（x²-y²）/（x+y）-2x-2y=（x+y）(x-y)/(x+y)-2x-2y
=x-y-2x-2y
=-x-3y
代入x=3，y=1/3，得（x²-y²）/（x+y）-2x-2y=-3-1=-4

Answer 4

（x²-y²）/（x+y）-2x-2y
=(x+y)(x-y)/(x+y)-2x-2y
=x-y-2x-2y
= -x-3y
当x=3，y=1/3时
上式= -3-3×1/3= -3-1= -4

Answer 5

解答：

(x²-y²）/（x+y）-2x-2y=[(x^2-y^2)-2(x+y)^2]/(x+y)
=(x^2-y^2-2x^2-4xy-2y^2)/(x+y)=-(x^2+4xy+3y^2)/(x+y)
=-(x+y)(x+3y)/(x+y)=-(x+3y)=-(3+1)=-4

Answer 6

（x²-y²）/（x+y）-2x-2y
=(x+y)(x-y)/（x+y）-2x-2y
=x-y-2x-2y
=-x-3y
x=3，y=1/3代入
-3-3(1/3)
=-3-1
=-4