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JEE PYQ: Quadratic Equation Question 22

Question 22 - 2020 (05 Sep Shift 2)

If $\alpha$ and $\beta$ are the roots of the equation, $7x^2 - 3x - 2 = 0$, the the value of $\frac{\alpha}{1-\alpha^2} + \frac{\beta}{1-\beta^2}$ is equal to:

(1) $\frac{27}{32}$

(2) $\frac{1}{24}$

(3) $\frac{3}{8}$

(4) $\frac{27}{16}$

Show Answer

Answer: (4)

Solution

Let $\alpha$ and $\beta$ be the roots of the quadratic equation $7x^2 - 3x - 2 = 0$

$\therefore \alpha + \beta = \frac{3}{7}$, $\alpha\beta = \frac{-2}{7}$

Now, $\frac{\alpha}{1-\alpha^2} + \frac{\beta}{1-\beta^2}$

$= \frac{\alpha - \alpha\beta(\alpha+\beta) + \beta}{1 - (\alpha^2 + \beta^2) + (\alpha\beta)^2}$

$= \frac{(\alpha+\beta) - \alpha\beta(\alpha+\beta)}{1 - (\alpha+\beta)^2 + 2\alpha\beta + (\alpha\beta)^2}$

$= \frac{\frac{3}{7} + \frac{2}{7} \cdot \frac{3}{7}}{1 - \frac{9}{49} + 2 \cdot \frac{-2}{7} + \frac{4}{49}} = \frac{27}{16}$


Learning Progress: Step 22 of 50 in this series