Verified Solution • 2025–26 Session moderate

QJEEM25Mac884: JEE Main 2025 Mathematics - Class 12 JEEM25Mac884 Solution: If , then the value of equals:

If lim⁡x→∞((e1−e)(1e−x1+x))x=α\lim_{x \to \infty} \left( \left( \frac{e}{1 - e} \right) \left( \frac{1}{e} - \frac{x}{1 + x} \right) \right)^x = \alpha , then the value of log⁡eα1+log⁡eα\frac{\log_e \alpha}{1 + \log_e \alpha} equals:

Multiple Choice Options

  • A e−1e^{-1}
  • B e−2e^{-2}
  • C e2\mathrm{e}^2
  • D ee

Step-by-Step Verified Solution

αamp;=lim⁡x→∞[(e1−e)(1e−x1+x)]x.\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

x1+x=1−11+x,\frac{x}{1+x} = 1 - \frac{1}{1+x},

so

1e−x1+x=1e−(1−11+x)=1e−1+11+x.\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

1−1e=e−1e,1 - \frac{1}{e} = \frac{e-1}{e},

to obtain

1e−x1+x=−e−1e+11+x.\frac{1}{e} - \frac{x}{1+x} = -\frac{e-1}{e} + \frac{1}{1+x}.

Multiplying by the prefactor, we have

(e1−e)(1e−x1+x)=e1−e[−e−1e+11+x].\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:

e1−e⋅(−e−1e)+e1−e⋅11+x=−e−11−e+e(1−e)(1+x).\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),1-e = -(e-1),

it follows that

−e−11−e=−e−1−(e−1)=1.-\frac{e-1}{1-e} = -\frac{e-1}{-(e-1)} = 1.

Hence, the expression simplifies to

1+e(1−e)(1+x).1 + \frac{e}{(1-e)(1+x)}.

Again, replacing 1−e1-e by −(e−1)-(e-1) ,

1+e−(e−1)(1+x)=1−e(e−1)(1+x).1 + \frac{e}{-(e-1)(1+x)} = 1 - \frac{e}{(e-1)(1+x)}.

Thus, the limit becomes

α=lim⁡x→∞(1−e(e−1)(1+x))x.\alpha = \lim_{x \to \infty} \left(1 - \frac{e}{(e-1)(1+x)}\right)^x.

For large xx , note that

1+x∼x,1+x \sim x,

so we approximate

α=lim⁡x→∞(1−e(e−1)x)x.\alpha = \lim_{x \to \infty} \left(1 - \frac{e}{(e-1)x}\right)^x.

Recall the limit

lim⁡x→∞(1−ax)x=e−a,\lim_{x \to \infty} \left(1 - \frac{a}{x}\right)^x = e^{-a},

with

a=ee−1.a = \frac{e}{e-1}.

Therefore,

α=e−ee−1.\alpha = e^{-\frac{e}{e-1}}.

Taking the natural logarithm,

ln⁡α=−ee−1.\ln \alpha = -\frac{e}{e-1}.

Now, compute

ln⁡α1+ln⁡α=−ee−11−ee−1.\frac{\ln \alpha}{1 + \ln \alpha} = \frac{-\frac{e}{e-1}}{1 - \frac{e}{e-1}}.

Express the denominator with a common denominator:

1−ee−1=e−1e−1−ee−1=−1e−1.1 - \frac{e}{e-1} = \frac{e-1}{e-1} - \frac{e}{e-1} = -\frac{1}{e-1}.

Thus,

−ee−1−1e−1=e1=e.\frac{-\frac{e}{e-1}}{-\frac{1}{e-1}} = \frac{e}{1} = e.

The final result is

ln⁡α1+ln⁡α=e.\frac{\ln\alpha}{1+\ln\alpha} = e.

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