Quadratic Equations
Formation of Equations
Grade 11
Question:
<p>If <span class="inline-math">a \neq b\</span> but <span class="inline-math">a^2 = 5a - 3\</span> and <span class="inline-math">b^2 = 5b - 3\</span>, then the equation with roots <span class="inline-math">\frac{a}{b}, \frac{b}{a}\</span> is:</p>
<p>(a) <span class="inline-math">3x^2 - 25x + 3 = 0\</span></p>
<p>(b) <span class="inline-math">x^2 + 5x - 3 = 0\</span></p>
<p>(c) <span class="inline-math">x^2 - 5x + 3 = 0\</span></p>
<p>(d) <span class="inline-math">3x^2 - 19x + 3 = 0\</span></p>
Step-by-Step Solution
Key Concept: Since both a and b satisfy the same quadratic equation x² = 5x - 3, they are roots of x² - 5x + 3 = 0. Use Vieta's formulas to find sum and product of a and b, then construct a new quadratic with roots a/b and b/a.
Step 1: Identify the equation satisfied by $a$ and $b$
Given $a^2 = 5a - 3$ and $b^2 = 5b - 3$.
Rearranging these equations, we obtain $a^2 - 5a + 3 = 0$ and $b^2 - 5b + 3 = 0$.
Since $a \neq b$, $a$ and $b$ are the distinct roots of the quadratic equation:
$$ x^2 - 5x + 3 = 0 $$
Step 2: Apply Vieta's formulas
For the quadratic equation $x^2 - 5x + 3 = 0$, Vieta's formulas state:
The sum of the roots is $a + b = -\frac{(-5)}{1} = 5$.
The product of the roots is $ab = \frac{3}{1} = 3$.
Step 3: Find the sum of the new roots
Let the new roots be $\alpha = \frac{a}{b}$ and $\beta = \frac{b}{a}$.
The sum of the new roots is:
$$ \alpha + \beta = \frac{a}{b} + \frac{b}{a} = \frac{a^2 + b^2}{ab} $$
To find $a^2 + b^2$, we use the identity $a^2 + b^2 = (a+b)^2 - 2ab$.
Substituting the values from Step 2:
$$ a^2 + b^2 = (5)^2 - 2(3) = 25 - 6 = 19 $$
Therefore, the sum of the new roots is:
$$ \alpha + \beta = \frac{19}{3} $$
Step 4: Find the product of the new roots
The product of the new roots is:
$$ \alpha \cdot \beta = \left(\frac{a}{b}\right) \cdot \left(\frac{b}{a}\right) = 1 $$
Step 5: Form the quadratic equation
A quadratic equation with roots $\alpha$ and $\beta$ is given by $x^2 - (\alpha + \beta)x + \alpha\beta = 0$.
Substituting the values from Step 3 and Step 4:
$$ x^2 - \left(\frac{19}{3}\right)x + 1 = 0 $$
To eliminate the fraction, multiply the entire equation by 3:
$$ 3x^2 - 19x + 3 = 0 $$
Correct Answer: A