已知x的平方+x-1=0,求x的三次方减二倍x+7的值

x^2 + x - 1 =0
x^2 + x =1
x^3 + x^2 =x → x^3 = x - x^2
x^3 - 2x + 7 代入
=x - x^2 - 2x +7
=-(x^2 + x) +7
=-1+7
=6