一元二次方程 x2+px+4=0x^{2} + px + 4 = 0 的根为 α2\alpha^{2} 和 β2\beta^{2},其中 α>0\alpha > 0,β>0\beta > 0 且 α>β\alpha > \beta。

  1. (a)

    求 α−β\alpha - \beta 关于 pp 的表达式。

  2. (b)

    在 p=−5p = -5 的情况下,求一个以 α−1\alpha - 1 和 1−β1 - \beta 为根的一元二次方程。

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完整解法

5(a)

1. Find the sum and product of the roots $\alpha^2$ and $\beta^2$.

Sum of roots: α2+β2=−p\alpha^2 + \beta^2 = -p
Product of roots: α2β2=4\alpha^2 \beta^2 = 4
Since α>0\alpha > 0 and β>0\beta > 0, αβ=4=2\alpha\beta = \sqrt{4} = 2.

2. Express $(\alpha - \beta)^2$ in terms of $p$.

(α−β)2=α2−2αβ+β2(\alpha - \beta)^2 = \alpha^2 - 2\alpha\beta + \beta^2
(α−β)2=(α2+β2)−2αβ(\alpha - \beta)^2 = (\alpha^2 + \beta^2) - 2\alpha\beta
(α−β)2=−p−2(2)=−p−4(\alpha - \beta)^2 = -p - 2(2) = -p - 4

3. Find $\alpha - \beta$ by taking the square root.

Since α>β\alpha > \beta, α−β>0\alpha - \beta > 0.
Therefore, α−β=−p−4\alpha - \beta = \sqrt{-p - 4}.

答案

−p−4\sqrt{-p - 4}

5(b)

1. Substitute $p = -5$ into the original equation and solve for $\alpha^2$ and $\beta^2$.

When p=−5p = -5, the equation is x2−5x+4=0x^{2} - 5x + 4 = 0.
(x−4)(x−1)=0(x - 4)(x - 1) = 0
x=4 or x=1x = 4 \text{ or } x = 1
Since the roots are α2\alpha^2 and β2\beta^2, the values are 44 and 11.

2. Determine the exact values of $\alpha$ and $\beta$ using the given conditions.

Given α>β\alpha > \beta, we must have α2=4\alpha^2 = 4 and β2=1\beta^2 = 1.
Since α>0\alpha > 0 and β>0\beta > 0:
α=4=2\alpha = \sqrt{4} = 2
β=1=1\beta = \sqrt{1} = 1

3. Calculate the roots of the new quadratic equation.

The new roots are α−1\alpha - 1 and 1−β1 - \beta.
α−1=2−1=1\alpha - 1 = 2 - 1 = 1
1−β=1−1=01 - \beta = 1 - 1 = 0

4. Form the new quadratic equation.

Sum of new roots =1+0=1= 1 + 0 = 1
Product of new roots =1×0=0= 1 \times 0 = 0
The new quadratic equation is x2−(sum)x+(product)=0x^{2} - (\text{sum})x + (\text{product}) = 0:
x2−x=0x^{2} - x = 0

答案

x2−x=0x^{2} - x = 0

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