如果x2+x-1=0那么代数式x3+2x-7的值为

题目

如果x2+x-1=0那么代数式x3+2x-7的值为

答案

x3-2x-7
=x3+x2-x2+2x-7
=(x3+x2-x)-(x2+x-1)-8
=x(x2+x-1)-(x2+x-1)-8
=-8