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JEE Advanced · Mathematics · 22. Functions

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If a continuous \(f\) defined on the real line \(R\), assume positive and negative values in \(R\), then the equation \(f(x)=0\) has a root in \(R\). For example, if it is known that a continuous function \(f\) on \(R\) is positive at some point and its minimum values is negative, then the equation \(f(x)=0\) has a root in \(R\).
Consider \(f(x)=k e^x-x\) for all real \(x\), where \(k\) is real constant.Question:
For \(k>0\), the set of all values of \(k\) for which \(k e^x-x=0\) has two distinct roots, is

  1. A
    \(\left(0, \frac{1}{e}\right)\)
  2. B
    \(\left(\frac{1}{e}, 1\right)\)
  3. C
    \(\left(\frac{1}{e}, \infty\right)\)
  4. D
    \((0,1)\)
Verified Solution

Answer & Solution

Correct Answer

(A)
\(\left(0, \frac{1}{e}\right)\)

Step-by-step Solution

Detailed explanation

For two distinct roots \(1+\ln k < 0(k>0)\)
\(\ln k < -1 \quad k < \frac{1}{e}\)
Hence, \(k \in\left(0, \frac{1}{e}\right)\)
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