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Use: $4x=3x+x$ and $-4x=-3x-x$ $(x^2+3)^2-16x^2=(x^2+3)^2-(4x)^2=(x^2+3+4x)(x^2+3-4x)=(x^2+3x+x+3)(x^2-3x-x+3)=[x(x+3)+1(x+3)][x(x-3)-1(x-3)]=(x+1)(x+3)(x-1)(x-3)$
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