(x+y)/(x^4-y^4)÷[1/(x^2+y^2)]
=(x+y)/[(x^2+y^2)(x+y)(x-y)]÷[1/(x^2+y^2)]
=1/[(x^2+y^2)(x-y)]*(x^2+y^2)
=1/(x-y)x^4-y^4=(x^2+y^2)(x+y)(x-y)?????????????????这是平方差公式，并且是两次使用x^4-y^4=(x²)²-(y²)²=(x²+y²)(x²-y²)=(x²+y²)(x+y)(x-y)