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JEE Main 20268 AprilEvening Shift
Exam
JEE Main
Year
2026
Date
8 April
Shift
Evening Shift
Subject
Mathematics
Chapter
Functions

JEE Main 2026 · 8 April · Evening Shift · Mathematics · Functions

Let f:(1,∞)→ℝ be a function defined as f(x) = (x-1)/(x+1) . Let f^...

Let f:(1,∞)→Rf:(1,\infty)\to\mathbb{R} be a function defined as f(x)=x−1x+1f(x) = \dfrac{x-1}{x+1}. Let fi+1(x)=f(fi(x))f^{i+1}(x) = f(f^i(x)), i=1,2,…,25i=1, 2, \ldots, 25, where f1(x)=f(x)f^1(x)=f(x). If g(x)+f26(x)=0g(x) + f^{26}(x) = 0, x∈(1,∞)x \in (1, \infty), then the area of the region bounded by the curves y=g(x)y=g(x), 2y=2x−32y=2x-3, y=0y=0 and x=4x=4 is:

Options

  1. A18+log⁡e2\dfrac{1}{8} + \log_e 2
  2. B14+log⁡e2\dfrac{1}{4} + \log_e 2
  3. C56+3log⁡e2\dfrac{5}{6} + 3\log_e 2
  4. D56+log⁡e2\dfrac{5}{6} + \log_e 2

Correct answer

A. 18+log⁡e2\dfrac{1}{8} + \log_e 2

Step-by-step solution

Given f(x)=x−1x+1f(x) = \dfrac{x-1}{x+1}. Let us find the first few compositions of f(x)f(x):

f2(x)=f(f(x))=x−1x+1−1x−1x+1+1=x−1−x−1x−1+x+1=−22x=−1xf^2(x) = f(f(x)) = \dfrac{\dfrac{x-1}{x+1}-1}{\dfrac{x-1}{x+1}+1} = \dfrac{x-1-x-1}{x-1+x+1} = \dfrac{-2}{2x} = -\dfrac{1}{x}

f3(x)=f(f2(x))=f(−1x)=−1x−1−1x+1=−1−x−1+x=x+11−xf^3(x) = f(f^2(x)) = f\left(-\dfrac{1}{x}\right) = \dfrac{-\dfrac{1}{x}-1}{-\dfrac{1}{x}+1} = \dfrac{-1-x}{-1+x} = \dfrac{x+1}{1-x}

f4(x)=f(f3(x))=f(x+11−x)=x+11−x−1x+11−x+1=x+1−1+xx+1+1−x=2x2=xf^4(x) = f(f^3(x)) = f\left(\dfrac{x+1}{1-x}\right) = \dfrac{\dfrac{x+1}{1-x}-1}{\dfrac{x+1}{1-x}+1} = \dfrac{x+1-1+x}{x+1+1-x} = \dfrac{2x}{2} = x

Since f4(x)=xf^4(x) = x, the sequence of functions is periodic with a period of 44.

Therefore, f26(x)=f4×6+2(x)=f2(x)=−1xf^{26}(x) = f^{4 \times 6 + 2}(x) = f^2(x) = -\dfrac{1}{x}.

We are given g(x)+f26(x)=0g(x) + f^{26}(x) = 0, which implies:
g(x)−1x=0⇒g(x)=1xg(x) - \dfrac{1}{x} = 0 \Rightarrow g(x) = \dfrac{1}{x}

We need to find the area of the region bounded by the curves y=1xy = \dfrac{1}{x}, y=x−32y = x - \dfrac{3}{2} (from 2y=2x−32y = 2x - 3), y=0y = 0, and x=4x = 4.

First, find the point of intersection of y=1xy = \dfrac{1}{x} and y=x−32y = x - \dfrac{3}{2}:
1x=x−32⇒2x2−3x−2=0\dfrac{1}{x} = x - \dfrac{3}{2} \Rightarrow 2x^2 - 3x - 2 = 0
(2x+1)(x−2)=0(2x+1)(x-2) = 0
Since x∈(1,∞)x \in (1, \infty), we get x=2x = 2.

The line y=x−32y = x - \dfrac{3}{2} intersects the x-axis (y=0y = 0) at x=32x = \dfrac{3}{2}.

The required area AA is bounded by y=x−32y = x - \dfrac{3}{2} from x=32x = \dfrac{3}{2} to x=2x = 2, and by y=1xy = \dfrac{1}{x} from x=2x = 2 to x=4x = 4.

A=∫3/22(x−32)dx+∫241xdxA = \int_{3/2}^{2} \left(x - \dfrac{3}{2}\right) dx + \int_{2}^{4} \dfrac{1}{x} dx

Evaluating the first integral:
∫3/22(x−32)dx=[12(x−32)2]3/22=12(2−32)2−0=12(12)2=18\int_{3/2}^{2} \left(x - \dfrac{3}{2}\right) dx = \left[ \dfrac{1}{2}\left(x - \dfrac{3}{2}\right)^2 \right]_{3/2}^{2} = \dfrac{1}{2} \left(2 - \dfrac{3}{2}\right)^2 - 0 = \dfrac{1}{2} \left(\dfrac{1}{2}\right)^2 = \dfrac{1}{8}

Evaluating the second integral:
∫241xdx=[log⁡ex]24=log⁡e4−log⁡e2=log⁡e2\int_{2}^{4} \dfrac{1}{x} dx = [\log_e x]_{2}^{4} = \log_e 4 - \log_e 2 = \log_e 2

Total Area = 18+log⁡e2\dfrac{1}{8} + \log_e 2
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