Q152 / 269 · NDA 2025 (Phase 1) · Mathematics
If one root of the equation $x^2 - kx + k = 0$ exceeds the other by $2\sqrt{3}$, then which one of the following is a value of $k$?
- A$3$
- Correct answerB$6$
- C$9$
- D$12$
Solution
Let the roots of the equation $x^2 - kx + k = 0$ be $\alpha$ and $\beta$.
Given that one root exceeds the other by $2\sqrt{3}$, we have $|\alpha - \beta| = 2\sqrt{3}$.
Squaring both sides, we get $(\alpha - \beta)^2 = 12$.
Using the identity $(\alpha - \beta)^2 = (\alpha + \beta)^2 - 4\alpha\beta$, we can substitute the sum and product of the roots.
From the given quadratic equation, $\alpha + \beta = k$ and $\alpha\beta = k$.
Substituting these values, we get $k^2 - 4k = 12$.
$k^2 - 4k - 12 = 0$
$(k - 6)(k + 2) = 0$
Thus, $k = 6$ or $k = -2$.
Among the given options, $6$ is a value of $k$.
Answer: $6$
Given that one root exceeds the other by $2\sqrt{3}$, we have $|\alpha - \beta| = 2\sqrt{3}$.
Squaring both sides, we get $(\alpha - \beta)^2 = 12$.
Using the identity $(\alpha - \beta)^2 = (\alpha + \beta)^2 - 4\alpha\beta$, we can substitute the sum and product of the roots.
From the given quadratic equation, $\alpha + \beta = k$ and $\alpha\beta = k$.
Substituting these values, we get $k^2 - 4k = 12$.
$k^2 - 4k - 12 = 0$
$(k - 6)(k + 2) = 0$
Thus, $k = 6$ or $k = -2$.
Among the given options, $6$ is a value of $k$.
Answer: $6$
Hence, the correct answer is option: 'B'.