Differential Calculus
Differential Calculus
star_batch_jee_advanced_2025
Grade 12

Question:

MATCH THE FOLLOWING: Column 1: (A) $f(x) = \begin{cases} x\left(\left[\frac{1}{x}\right] + \left[\frac{2}{x}\right] + \cdots + \left[\frac{8}{x}\right]\right), & x \neq 0 \\ 9k, & x = 0 \end{cases}$ The value of $k$ such that $f$ is continuous at $x = 0$ is (B) $f(x) = \begin{cases} \left(1 + xe^{-1/x^2}\sin\frac{1}{x^3}\right)^{1/x^2}, & x \neq 0 \\ k, & x = 0 \end{cases}$ The value of $k$ such that $f$ is continuous at $x = 0$ is (C) $f: [0, \infty) \to \mathbb{R}; f(x) = \begin{cases} \left(2\sin\sqrt{x} + \sqrt{x}\sin\frac{1}{x}\right)^x, & x > 0 \\ k, & x = 0 \end{cases}$ The value of $k$ such that $f$ is continuous at $x = 0$ is (D) $f: (0, \pi) \to \mathbb{R}; f(x) = \begin{cases} \left(\frac{1 - \sin x}{(\pi - 2x)^2} \cdot \ln(1 + x^2 - 4\pi x + 4x^2)\right)^{\tan x}, & x \neq \frac{\pi}{2} \\ k, & x = \frac{\pi}{2} \end{cases}$ The value of $8\sqrt[4]{k}$ such that $f$ is continuous at $x = \frac{\pi}{2}$ is Column 2: (p) 1 (q) 2 (r) 3 (s) 4

Step-by-Step Solution

Key Concept: Sandwich theorem combined with floor function properties bounds the sum of floor functions between linear expressions whose limits are equal.
Using sandwich theorem, we establish that $x\left(\frac{1+2+3+\ldots+8}{x}-8\right)<\left[\frac{1}{x}\right]+\left[\frac{2}{x}\right]+\ldots+\left[\frac{8}{x}\right]\leq x\left(\frac{1+2+3+\ldots+8}{x}\right)$. Taking limits as $x\to 0$: $\lim_{x\to 0}\left(\left[\frac{1}{x}\right]+\left[\frac{2}{x}\right]+\ldots+\left[\frac{8}{x}\right]\right)=36$.
Correct Answer: [A-s] [B-p] [C-p] [D-p]

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