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Polynomials
RD Sharma
CBSE
Grade 10

Question:

If $\alpha, \beta$ are zeroes of $f(x) = a x^2 + b x + c$, form a quadratic polynomial whose zeroes are $\alpha + \dfrac{1}{\beta}$ and $\beta + \dfrac{1}{\alpha}$.

Step-by-Step Solution

Key Concept: $\alpha + \beta = -b/a, \alpha \beta = c/a$.<br>New Sum $= (\alpha + \beta) + \dfrac{\alpha + \beta}{\alpha \beta} = -\dfrac{b}{a} + \dfrac{-b/a}{c/a} = -\dfrac{b}{a} - \dfrac{b}{c} = -b \left(\dfrac{a + c}{ac}\right)$.<br>New Product $= \left(\alpha + \dfrac{1}{\beta}\right)\left(\beta + \dfrac{1}{\alpha}\right) = \alpha \beta + 1 + 1 + \dfrac{1}{\alpha \beta} = \dfrac{c}{a} + 2 + \dfrac{a}{c} = \dfrac{(a + c)^2}{ac}$.
New Sum $= -\dfrac{b}{a} - \dfrac{b}{c} = -\dfrac{b(a + c)}{ac}$. [2.0 Marks]
New Product $= \dfrac{c}{a} + 2 + \dfrac{a}{c} = \dfrac{c^2 + 2ac + a^2}{ac} = \dfrac{(a + c)^2}{ac}$. [2.0 Marks]
Polynomial is $x^2 + \dfrac{b(a + c)}{ac} x + \dfrac{(a + c)^2}{ac}$ or $ac x^2 + b(a + c)x + (a + c)^2$. [1.0 Mark]

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🎯 Official CBSE Marking Scheme:
Evaluating new sum $= -b(a+c)/(ac)$: 2.0 Marks
Evaluating new product $= (a+c)^2/(ac)$: 2.0 Marks
Forming polynomial $ac x^2 + b(a+c)x + (a+c)^2$: 1.0 Mark

Correct Answer:
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