已知x^2n=2,求(3x^3n)²-4(x²）^2n的值

题目

已知x^2n=2,求(3x^3n)²-4(x²）^2n的值

答案

∵x^(2n)=2
∴(3x^3n)²-4(x²）^2n
=3²x^(2×3n)-4*[x^(2n)]^2
=9*[x^(2n)]^3-4*[x^(2n)]^2
=9*2^3-4*2^2
=72-16
=56