Quadratic Equations
Equations with Transformed Roots
Grade 11
Question:
<p>If <i>α, β</i> are the roots of \(x^2 - 3x + 1 = 0\), then the equation whose roots are \(\frac{1}{\alpha + 2}\), \(\frac{1}{\beta + 2}\), is</p>
<p>(a) \(x^2 + x - 1 = 0\)</p>
<p>(b) \(x^2 + x + 1 = 0\)</p>
<p>(c) \(x^2 - x - 1 = 0\)</p>
<p>(d) None of these</p>
Step-by-Step Solution
Key Concept: If we know the roots of the original equation, we can find the sum and product of the new roots using Vieta's formulas, then construct the new quadratic equation. Alternatively, use substitution: if y = 1/(x+2), then x = 1/y - 2, and substitute into the original equation.
**Step 1:** Identify properties of the given roots.
Let $\alpha$ and $\beta$ be the roots of the equation $x^2 - 3x + 1 = 0$.
By Vieta's formulas, the sum of the roots is $\alpha + \beta = -(-3)/1 = 3$, and the product of the roots is $\alpha\beta = 1/1 = 1$.
**Step 2:** Define the new roots.
Let the new roots be $y_1 = \frac{1}{\alpha + 2}$ and $y_2 = \frac{1}{\beta + 2}$.
**Step 3:** Calculate the sum of the new roots.
The sum of the new roots is:
$$y_1 + y_2 = \frac{1}{\alpha + 2} + \frac{1}{\beta + 2}$$
$$y_1 + y_2 = \frac{(\beta + 2) + (\alpha + 2)}{(\alpha + 2)(\beta + 2)}$$
$$y_1 + y_2 = \frac{\alpha + \beta + 4}{\alpha\beta + 2\alpha + 2\beta + 4}$$
$$y_1 + y_2 = \frac{\alpha + \beta + 4}{\alpha\beta + 2(\alpha + \beta) + 4}$$
Substitute the values from Step 1:
$$y_1 + y_2 = \frac{3 + 4}{1 + 2(3) + 4} = \frac{7}{1 + 6 + 4} = \frac{7}{11}$$
**Step 4:** Calculate the product of the new roots.
The product of the new roots is:
$$y_1 y_2 = \left(\frac{1}{\alpha + 2}\right) \left(\frac{1}{\beta + 2}\right)$$
$$y_1 y_2 = \frac{1}{(\alpha + 2)(\beta + 2)}$$
$$y_1 y_2 = \frac{1}{\alpha\beta + 2(\alpha + \beta) + 4}$$
Substitute the values from Step 1:
$$y_1 y_2 = \frac{1}{1 + 2(3) + 4} = \frac{1}{1 + 6 + 4} = \frac{1}{11}$$
**Step 5:** Formulate the new quadratic equation.
A quadratic equation with roots $y_1$ and $y_2$ is given by $y^2 - (y_1 + y_2)y + y_1 y_2 = 0$.
Substitute the sum and product calculated in Steps 3 and 4:
$$y^2 - \left(\frac{7}{11}\right)y + \left(\frac{1}{11}\right) = 0$$
Multiply the entire equation by 11 to clear the denominators:
$$11y^2 - 7y + 1 = 0$$
Thus, the equation whose roots are $\frac{1}{\alpha + 2}$ and $\frac{1}{\beta + 2}$ is $11x^2 - 7x + 1 = 0$.
Correct Answer: A