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GATE CSE 2024 Session 1 Q11: Points where max{x, x³} is not differentiable

Question

Let f:R→Rf: \mathbb{R} \to \mathbb{R} be a function such that f(x)=max⁡{x,x3}f(x) = \max\{x, x^3\}, x∈Rx \in \mathbb{R}, where R\mathbb{R} is the set of all real numbers. The set of all points where f(x)f(x) is NOT differentiable is

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Answer: D (official GATE 2024 final answer key)

Concept used: The max of two smooth functions has corners where they cross with different slopes.

First step: Solve x=x3x = x^3 and check the slopes 11 and 3x23x^2 at each crossing.

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Source: official GATE 2024 Session 1 CS question paper and final answer key, organised by IISc Bangalore. Question paper · Answer key