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Let x, y be real numbers such that x gt ...

Let x, y be real numbers such that `x gt 2y gt 0` and
`2log (x-2y) = log x + log y`.
Then the possible values (s) of `(x)/(y)`

A

is 1 only

B

are 1 and 4

C

is 4 only

D

is 8 only

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The correct Answer is:
To solve the problem, we need to find the possible values of \( \frac{x}{y} \) given the conditions \( x > 2y > 0 \) and the equation \( 2 \log(x - 2y) = \log x + \log y \). ### Step-by-Step Solution: 1. **Start with the given equation:** \[ 2 \log(x - 2y) = \log x + \log y \] 2. **Use the properties of logarithms:** The right-hand side can be combined using the product property of logarithms: \[ \log x + \log y = \log(xy) \] Therefore, we can rewrite the equation as: \[ 2 \log(x - 2y) = \log(xy) \] 3. **Exponentiate both sides:** To eliminate the logarithm, we exponentiate both sides: \[ (x - 2y)^2 = xy \] 4. **Expand the left-hand side:** \[ x^2 - 4xy + 4y^2 = xy \] 5. **Rearrange the equation:** Move all terms to one side: \[ x^2 - 5xy + 4y^2 = 0 \] 6. **This is a quadratic equation in \( x \):** We can apply the quadratic formula where \( a = 1, b = -5y, c = 4y^2 \): \[ x = \frac{-(-5y) \pm \sqrt{(-5y)^2 - 4 \cdot 1 \cdot 4y^2}}{2 \cdot 1} \] Simplifying this gives: \[ x = \frac{5y \pm \sqrt{25y^2 - 16y^2}}{2} \] \[ x = \frac{5y \pm \sqrt{9y^2}}{2} \] \[ x = \frac{5y \pm 3y}{2} \] 7. **Calculate the two possible values for \( x \):** - First case: \[ x = \frac{8y}{2} = 4y \] - Second case: \[ x = \frac{2y}{2} = y \] 8. **Evaluate the conditions:** We have two cases for \( x \): - If \( x = 4y \), then \( \frac{x}{y} = 4 \). - If \( x = y \), then \( \frac{x}{y} = 1 \). 9. **Check the inequality \( x > 2y \):** - For \( x = 4y \): \( 4y > 2y \) (valid) - For \( x = y \): \( y > 2y \) (invalid) Thus, the only valid solution is: \[ \frac{x}{y} = 4 \] ### Final Answer: The possible value of \( \frac{x}{y} \) is \( 4 \).
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KVPY PREVIOUS YEAR-KVPY 2021-PART-I Mathematics
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