题目

1.若x^2 - 3x -1=0,求代数式2x^3 - 3x^2 -11x +8的值．
2.若3x^2 - x=1,求代数式6x^3 +7x^2 -5x +1999的值．

提问时间：2020-08-02

答案

x^2 - 3x -1=0
x^2-3x=1
2x^3 - 3x^2 -11x +8
=2x(x^2-3x)+3x^2-11x+8
=2x+3x^2-11x+8
=3x^2-9x+8
=3(x^2-3x)+8
=3+8
=11
6x^3 +7x^2 -5x +1999
=2x(3x^2-x)+9x^2-5x+1999
=2x+9x^2-5x+1999
=9x^2-3x+1999
=3(3x^2-x)+1999
=3+1999
=2002