已知x=4-3,求x4-6x3-2x2+18x+23x2-8x+15的值.

问题描述:

已知x=4-

3
,求
x4-6x3-2x2+18x+23
x2-8x+15
的值.

已知得(x-4)2=3,即x2-8x+13=0,则x2-8x=-13.
分子x4-6x3-2x2+18x+23,
=x4-8x3+2x3-2x2+18x+23,
=x2(x2-8x)+2x3-2x2+18x+23,
=-13x2+2x3-2x2+18x+23,
=2x3-16x2+x2+18x+23,
=2x(x2-8x)+x2+18x+23,
=-26x+x2+18x+23,
=x2-8x+23,
=-13+23,
=10,
分母是x2-8x+15=-13+15=2,

x4-6x3-2x2+18x+23
x2-8x+15
=
10
2
=5.
故答案为:5.