JEE MAIN - Mathematics (2025 - 22nd January Evening Shift - No. 14)
Explanation
$$ \begin{aligned} \alpha &= \lim_{x \to \infty} \left[\left(\frac{e}{1-e}\right)\left(\frac{1}{e}-\frac{x}{1+x}\right)\right]^x. \end{aligned} $$
Begin by rewriting the expression inside the limit. Notice that
$$ \frac{x}{1+x} = 1 - \frac{1}{1+x}, $$
so
$$ \frac{1}{e} - \frac{x}{1+x} = \frac{1}{e} - \left(1 - \frac{1}{1+x}\right) = \frac{1}{e} - 1 + \frac{1}{1+x}. $$
Express the constant part as
$$ 1 - \frac{1}{e} = \frac{e-1}{e}, $$
to obtain
$$ \frac{1}{e} - \frac{x}{1+x} = -\frac{e-1}{e} + \frac{1}{1+x}. $$
Multiplying by the prefactor, we have
$$ \left(\frac{e}{1-e}\right)\left(\frac{1}{e} - \frac{x}{1+x}\right) = \frac{e}{1-e}\left[-\frac{e-1}{e} + \frac{1}{1+x}\right]. $$
Split the expression:
$$ \frac{e}{1-e}\cdot\left(-\frac{e-1}{e}\right) + \frac{e}{1-e}\cdot\frac{1}{1+x} = -\frac{e-1}{1-e} + \frac{e}{(1-e)(1+x)}. $$
Since
$$ 1-e = -(e-1), $$
it follows that
$$ -\frac{e-1}{1-e} = -\frac{e-1}{-(e-1)} = 1. $$
Hence, the expression simplifies to
$$ 1 + \frac{e}{(1-e)(1+x)}. $$
Again, replacing $1-e$ by $-(e-1)$,
$$ 1 + \frac{e}{-(e-1)(1+x)} = 1 - \frac{e}{(e-1)(1+x)}. $$
Thus, the limit becomes
$$ \alpha = \lim_{x \to \infty} \left(1 - \frac{e}{(e-1)(1+x)}\right)^x. $$
For large $x$, note that
$$ 1+x \sim x, $$
so we approximate
$$ \alpha = \lim_{x \to \infty} \left(1 - \frac{e}{(e-1)x}\right)^x. $$
Recall the limit
$$ \lim_{x \to \infty} \left(1 - \frac{a}{x}\right)^x = e^{-a}, $$
with
$$ a = \frac{e}{e-1}. $$
Therefore,
$$ \alpha = e^{-\frac{e}{e-1}}. $$
Taking the natural logarithm,
$$ \ln \alpha = -\frac{e}{e-1}. $$
Now, compute
$$ \frac{\ln \alpha}{1 + \ln \alpha} = \frac{-\frac{e}{e-1}}{1 - \frac{e}{e-1}}. $$
Express the denominator with a common denominator:
$$ 1 - \frac{e}{e-1} = \frac{e-1}{e-1} - \frac{e}{e-1} = -\frac{1}{e-1}. $$
Thus,
$$ \frac{-\frac{e}{e-1}}{-\frac{1}{e-1}} = \frac{e}{1} = e. $$
The final result is
$$ \frac{\ln\alpha}{1+\ln\alpha} = e. $$
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