数学问题--------平方差公式&完全平方公式

(x+2y-3)(x-2y-3)=[(x-3)+2y]*[(x-3)-2y]
(x+y)²(x-y)²-(x-y)(x+y）(x²+y²)
=(x^2-y^2)^2-(x^2-y^2)(x^2+y^2)
=(x^4-2x^2 *y^2+y^4)-(x^4-y^4)
=-2x^2*y^2+2y^42.(x+y)²(x-y)²-(x-y)(x+y）(x²+y²)=-(x-y)²(x-y)²-(x-y)(x+y)(x²+y²)①=-(x-y)^4-[(x²-y²)(x²+y²)]②=-（x-y)^4-[x^4-y^4]③对不对，如果对，怎么继续做，如果错了，哪里错了，第几步，请改正第一步就已经错了.(x+y)²(x-y)²-(x-y)(x+y）(x²+y²)=-(x-y)²(x-y)²-(x-y)(x+y)(x²+y²)应该是：=[(x+y)(x-y)]²-(x-y)(x+y)(x²+y²)①=(x^2-y^2)^2-(x^2-y^2)(x^2+y^2)=(x^4-2x^2 *y^2+y^4)-(x^4-y^4)=-2x^2*y^2+2y^4-2x²y²+2y^4等不等于=-2(x-y)(x+y)y²还有-2x²y²+2y^4一定对吗，我也求出是这个数一定对=-2x^2*y^2+2y^4=-2(x^2-y^2)*y^2=-2(x-y)(x+y)*y^2