已知x+1/x=3,求x³+x-³的值
问题描述：
已知x+1/x=3,求x³+x-³的值
答
答：
x+1/x=3
x³+1/x³
=(x+1/x)(x²-1+1/x²)
=3[(x+1/x)²-3]
=3*(3²-3)
=3*6
=18