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Question #41310

Difficulty: 8/10 | 🟢 Free

The Question

Let M(x)=(sin⁡xcos⁡xcos⁡(x−π6)sin⁡2xcos⁡2xcos⁡(2x−π6)sin⁡3xcos⁡3xcos⁡(3x−π6)+3sin⁡x)M(x) = \begin{pmatrix} \sin x & \cos x & \cos(x-\frac{\pi}{6}) \\ \sin 2x & \cos 2x & \cos(2x-\frac{\pi}{6}) \\ \sin 3x & \cos 3x & \cos(3x-\frac{\pi}{6}) + \sqrt{3}\sin x \end{pmatrix} and N(x)=(10001000sin⁡x)N(x) = \begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & \sin x \end{pmatrix}.

Let S={x∈[0,2π]:det⁡(M(x))≠0}S = \{ x \in [0, 2\pi] : \det(M(x)) \neq 0 \}. For x∈Sx \in S, define g(x)=det⁡(M(x)−1N(x))g(x) = \det(M(x)^{-1} N(x)).

A function h:[0,2π]→Rh: [0, 2\pi] \to \mathbb{R} is defined as: h(x)={lim⁡n→∞11+∣g(x)∣nif x∈S0if x∉Sh(x) = \begin{cases} \lim_{n \to \infty} \frac{1}{1 + |g(x)|^n} & \text{if } x \in S \\ 0 & \text{if } x \notin S \end{cases}

Find the number of points in [0,2π][0, 2\pi] where h(x)h(x) is discontinuous.

Options

  • A.

    3

  • B.

    4

  • C.

    5

  • D.

    7

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Monster

Ultra-high difficulty questions designed to test your limit, stamina, and deep conceptual understanding.

Concepts Behind This Question

Composition Continuity|Linear Dependency of Rows|Inversion and Determinant Product

Why This Question Is Tricky

Common Mistakes

The problem tests the continuity of a composite limit function involving matrix determinants.

The primary trap lies in the singular points of M(x)M(x).

Students correctly identify x∈{0,π,2π}x \in \{0, \pi, 2\pi\} as points where M(x)M(x) is non-invertible and g(x)g(x) is undefined.

A common reflex is to immediately count these as points of discontinuity (yielding 7 points).

However, evaluating the limit of the extended function h(x)h(x) as xx approaches these singular points reveals that ∣g(x)∣→∞|g(x)| \to \infty, causing h(x)→0h(x) \to 0.

This perfectly matches the piecewise definition, making them removable discontinuities that have been patched.

The true discontinuities only occur at the boundary condition ∣g(x)∣=1|g(x)| = 1.

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterContinuity and Differentiability - Class 12
ConceptComposition Continuity
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree