Question 1174749
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Factor it as follows


    x^4 - 16x^2 + 48 = (x^2-12)*(x^2-4) = {{{(x-2*sqrt(3))*(x+2*sqrt(3))*(x-2)*(x+2)}}}.


The roots are  {{{2sqrt(3)}}}, -{{{2*sqrt(3)}}}, 2 and -2.
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PLEASE STOP posting duplicates.