已知n为正整数,且(x^n)^2=9.求(1/3x^2n)^2 -3(x^2)^2n的值

已知n为正整数,且(x^n)^2=9.
(1/3x^2n)^2 -3(x^2)^2n的值,急!
=1/9x^(4n)-3x^(4n)
=1/9(x^n)^4-3(x^n)^4
=1/9×9²-3×9²
=1/729-243
=-242-728/729;
=-242又729分之728；
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