若n为正整数,且x^2n=5,求(x^3n)^2-13(x^2)^2n的值
问题描述：
若n为正整数,且x^2n=5,求(x^3n)^2-13(x^2)^2n的值
答
(x^3n)^2-13(x^2)^2n
=x^(6n)-13x^(4n)
=(x^2n)^3-13(x^2n)^2
=5^3-13*5^2
=125-13*25
=125-325
=-200