若n为正整数,且x^2n=7,求(3×x^3n)^2-13(x^2)^2的值
问题描述：
若n为正整数,且x^2n=7,求(3×x^3n)^2-13(x^2)^2的值
答
(3×x^3n)^2-13(x^2n)^2
=9(x^2n)³-13(x^2n)²
=9×7³-13×7²
=7²×(63-13)
=49×50
=2450