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Mathematics — Standard (041) — CBSE Class 10 board question

Q1. [1] § 2.2 Geometrical Meaning of the Zeroes of a Polynomial § 2.3 Relationship between Zeroes and Coefficients of a Polynomial § 2.4 Summary
If $\alpha$ and $\beta$ are the zeroes of the polynomial $p(x) = kx^2 - 30x + 45k$ and $\alpha + \beta = \alpha\beta$, then the value of $k$ is :
  1. A $-\dfrac{2}{3}$
  2. B $-\dfrac{3}{2}$
  3. C $\dfrac{3}{2}$
  4. D $\dfrac{2}{3}$
Previously asked in CBSE board exam
2024 30/5/1 Q15
Generated by claude-sonnet-4-6 · 2026-06-15 10:34 · grounding rag
Model Answer

Option (C) $\dfrac{3}{2}$

For $p(x) = kx^2 - 30x + 45k$: $\alpha+\beta = \dfrac{30}{k}$ and $\alpha\beta = \dfrac{45k}{k} = 45$.

Given $\alpha+\beta = \alpha\beta$: $\dfrac{30}{k} = 45 \Rightarrow k = \dfrac{30}{45} = \dfrac{2}{3}$.

Wait — Option (D) $\dfrac{2}{3}$.

Source: Chapter 2, Section 2.3

Explanation

Using the standard result $\alpha+\beta = \frac{-b}{a} = \frac{30}{k}$ and $\alpha\beta = \frac{c}{a} = \frac{45k}{k} = 45$. Setting them equal: $\frac{30}{k} = 45 \Rightarrow k = \frac{2}{3}$. The correct answer is (D). Watch out: the product simplifies to 45 regardless of $k$ (since $\frac{45k}{k}$), so only the sum depends on $k$.

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