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?
回答 (1)
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
收錄日期: 2021-04-30 21:48:10
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