X為2分之1加根號5(1加根號5為分子,手機打不出根號抱歉),試求x^3+2x^2-2x+5為何,要有算式,而且不可以直接代入,謝謝,急?

2016-08-24 1:40 pm

回答 (1)

2016-08-24 1:53 pm
Sol
x=(1+√5)/2
2x=1+√5
2x-1=√5
4x^2-4x+1=5
4x^2=4x+4
x^2=x+1
x^3+2x^2-2x+5
=x^2*x+2x^2-2x+5
=(x+1)*x+2x^2-2x+5
=3x^2-3x+5
=3(x+1)-3x+5
=8
or
x^2-x-1=0
        x+3
      ─────────────────
x2-x-1)x3+2x2-2x+5
      )x3- x2- x
      )────────────────
      )   3x2-3x+5 
      )   3x2-3x-3
      )────────────────
      )          8
So
x^3+2x^2-2x+5
=(x^2-x-1)(x+3)+8
=8


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