已知 3x2−x+2=03x^2 - x + 2 = 0 的根为 α\alpha 和 β\beta,求一个以 α+1β\frac{\alpha + 1}{\beta} 和 β+1α\frac{\beta + 1}{\alpha} 为根的一元二次方程。

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完整解法

1

1. Find the sum and product of the original roots $\alpha$ and $\beta$.

From 3x2−x+2=03x^2 - x + 2 = 0:
α+β=−−13=13\alpha + \beta = -\frac{-1}{3} = \frac{1}{3}
αβ=23\alpha\beta = \frac{2}{3}

2. Express the sum of the new roots in terms of $\alpha + \beta$ and $\alpha\beta$.

Sum of new roots =α+1β+β+1α= \frac{\alpha + 1}{\beta} + \frac{\beta + 1}{\alpha}
=α(α+1)+β(β+1)αβ= \frac{\alpha(\alpha + 1) + \beta(\beta + 1)}{\alpha\beta}
=α2+α+β2+βαβ= \frac{\alpha^2 + \alpha + \beta^2 + \beta}{\alpha\beta}
=(α+β)2−2αβ+(α+β)αβ= \frac{(\alpha + \beta)^2 - 2\alpha\beta + (\alpha + \beta)}{\alpha\beta}

3. Calculate the sum of the new roots.

Sum =(13)2−2(23)+1323= \frac{(\frac{1}{3})^2 - 2(\frac{2}{3}) + \frac{1}{3}}{\frac{2}{3}}
=19−43+1323= \frac{\frac{1}{9} - \frac{4}{3} + \frac{1}{3}}{\frac{2}{3}}
=−8923= \frac{-\frac{8}{9}}{\frac{2}{3}}
=−43= -\frac{4}{3}

4. Express and calculate the product of the new roots.

Product of new roots =(α+1β)(β+1α)= \left(\frac{\alpha + 1}{\beta}\right)\left(\frac{\beta + 1}{\alpha}\right)
=αβ+α+β+1αβ= \frac{\alpha\beta + \alpha + \beta + 1}{\alpha\beta}
=23+13+123= \frac{\frac{2}{3} + \frac{1}{3} + 1}{\frac{2}{3}}
=223= \frac{2}{\frac{2}{3}}
=3= 3

5. Form the new quadratic equation.

x2−(Sum of roots)x+(Product of roots)=0x^2 - (\text{Sum of roots})x + (\text{Product of roots}) = 0
x2−(−43)x+3=0x^2 - \left(-\frac{4}{3}\right)x + 3 = 0
x2+43x+3=0x^2 + \frac{4}{3}x + 3 = 0
3x2+4x+9=03x^2 + 4x + 9 = 0

答案

3x2+4x+9=03x^2 + 4x + 9 = 0

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