八年级计算题

楼上的计算错误啊 ！

1： =ab(a^2-1)-ab
=ab(a^2-2)

2: =x^4-4y^4-x^2(x^2-4y^4)
=x^4-4x^2y^2+4y^4-x^4+4x^2y^4
=4y^4

3. =[3x^2-2x^2-2xy+2xy-y^2]÷(x+y)
=(x^2-y^2)÷(x+y)
=(x+y)(x-y)÷(x+y)
=x-y
当 x=1/2 , y=1/3时，原式=1/2-1/3=1/6

1.ab(a+1)(a-1)-ab
=ab(a^2-1）-ab=ab(a^2-2)
2原式=.(x^2-2y^2)^2-x^2(x^2-y^2)=(x^4-4x^2y^2+4y^4)-x^4+x^2y^2
=-3x^2y^2+4y^4=y^2(4y^2-3x^2)

3.原式=[3x^2-2x^2-2xy+2xy-y^2]/x+y
=[x^2-y^2]/x+y=(x+y)(x-y)/(x+y)=x-y=1/6
几题都用到平方差公式

ab(a+1)(a-1)-ab=ab(a^2-1)-ab=ba^3-2ab

(x^2-2y^2)^2-x^2(x+2y)(x-2y)
=x^4-4x^2y^2+4y^4-[x^2(x^2-4y^2)]
=4y^4

[3x^2-2x(x+y)+y(2x-y)]÷(x+y)
=(x^2-y^2)/(x+y)
=x-y
=1/6

1.原式= ab(a^2-1）-ab
=ab(a^2-2)

2.原式=x^4-4y^4-x^2(x^2-4y^4)
=x^4-4x^2y^2+4y^4-x^4+4x^2y^4
=4y^4

3.原式=[3x^2-2x^2-2xy+2xy-y^2]/x+y