Answer
$x^4-y^4$
Work Step by Step
We will use the result that $(a-b)(a+b)=a^2-b^2$
Hence,
$(x+y)(x-y)(x^2+y^2)=(x^2-y^2)(x^2+y^2)$ (pairing the first two product terms and applying the result mentioned above)
$(x^2-y^2)(x^2+y^2)=x^4-y^4$
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