答：
x^2-4x=1
两边同除以x得：
x-4=1/x
x-1/x=4
x^3-1/x^3
=(x-1/x)(x^2+1+1/x^2)
=3*[(x-1/x)^2+3]
=3*(3^2+3)
=3*12
=36