由x^(2n)=2得(x^n)²=2,x^n=±√2
-3x^(3n)-4(-x²)^(2n)=-3(x^n)^3-4(x^n)^4=-3(√2)^3-4(√2)^4或=-3(-√2)^3-4(-√2)^4
即 ±6√2-16