教育/科学

若x&sup2+x若x²+x-1=0,那么代数式x三次

2012-10-26 18:13:28寒***

若x²+x-1=0,那么代数式x三次方+2x²-7的值为_____请详细写出过程若x&sup2+x若x²+x-1=0,那么代数式x三次方+2x²-7的值为_____请详细写出过程：解：x²+x-1=0 ==> ?

解：x²+x-1=0 ==> x²=1-x `x³+2x²-7 =x*x²+2(1-x)-7 =x(1-x)+2(1-x)-7 =-x-x²-5 =-x-(1-x)-5 =-6

2012-10-26 18:34:44

x³+2x²-7 =x³+x²-x+x²+x-1-6 =x(x²+x-1)+(x²+x-1)-6 =-6 (∵x²+x-1=0)

2012-10-26 18:52:17

由于x^2+x-1=0,所以x^2=1-x x^3+2x^2-7 =x^2*x+2x^2-7 =(1-x)x+2x^2-7 =x-x^2+2x^2-7 =x^2+x-7 =x^2+x-1-6 =0-6 =-6

2012-10-26 18:45:40

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

2012-10-26 18:42:56

很赞哦！ (100)