Answer
$x^4-2x^2+1$
Work Step by Step
Use the formula $(a+b)(a-b)=a^2-b^2$, we have
$(x+1)(x+1)(x-1)(x-1)=(x+1)(x-1)[(x+1)(x-1)]=(x^2-1)(x^2-1)=(x^2-1)^2=x^4-2x^2+1$
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