MCQMediumJEE 2024Differentiability

JEE Mathematics 2024 Question with Solution

Consider the function ff: (0,2)(0,2)RR defined by f(x)=x2+2xf(x) = x^2 + \frac{2}{x}, and the function g(x)g(x) by min{f(t)}\min\{f(t)\} for 0<tx,  0<x10 < t \le x, \; 0 < x \le 1 and g(x)=(32+x)g(x) = \left(\frac{3}{2} + x\right) for 1<x<21 < x < 2. Which statement about gg is correct?

  • A

    gg is continuous but not differentiable at x=1x = 1

  • B

    gg is not continuous for all xx in (0,2)(0,2)

  • C

    gg is neither continuous nor differentiable at x=1x = 1

  • D

    gg is continuous and differentiable for all xx in (0,2)(0,2)

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: f(x)=x2+2xf(x) = \frac{x}{2} + \frac{2}{x} and

g(x)=min{f(t):0g(x)=\min\{f(t):0

From the solution, for 00

Hence on (0,x](0,x] the minimum is attained at the endpoint, and as used in the source solution,

g(1)=12+2=52g(1)=\frac{1}{2}+2=\frac{5}{2}

So for the left side near x=1x=1, the value approaches 52\frac{5}{2}.

For 11

Continuity and differentiability check

Now compare the one-sided limits at x=1x=1:

limx1g(x)=52,limx1+g(x)=52\lim_{x\to 1^-} g(x)=\frac{5}{2}, \qquad \lim_{x\to 1^+} g(x)=\frac{5}{2}

These are equal and match the value at x=1x=1 as concluded in the source solution.

So, gg is continuous at x=1x=1.

For differentiability, the source solution states that the expressions on the two sides of x=1x=1 are different, so the left-hand derivative and right-hand derivative do not match. Therefore, gg is not differentiable at x=1x=1.

Therefore, the correct option is A: gg is continuous but not differentiable at x=1x=1.

Note: The question statement gives f(x)=x2+2xf(x)=x^2+\frac{2}{x}, but the solution works with f(x)=x2+2xf(x)=\frac{x}{2}+\frac{2}{x}. The extracted answer follows the solution, which explicitly concludes that the correct option is A.

Common mistakes

  • Using the question expression for f(x)f(x) and ignoring that the solution works with a different function. Since answer extraction must follow the solution, the conclusion should be based on the worked function used there.

  • Checking only continuity and not differentiability at x=1x=1. Even if the left-hand limit and right-hand limit match, differentiability still requires matching one-sided derivatives.

  • Assuming that a piecewise-defined function is automatically non-continuous at the junction point. Continuity depends on the function values and one-sided limits, not merely on having different formulas.

Practice more Differentiability questions

Get unlimited AI-adaptive practice, mastery tracking, and an AI tutor that explains every step — free to start.

Related questions