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Question #41370

Difficulty: 8/10 | 🟢 Free

The Question

Let a⃗=ti^+t2j^+k^\vec{a} = t \hat{i} + t^2 \hat{j} + \hat{k}, b⃗=2i^+4j^+k^\vec{b} = 2 \hat{i} + 4 \hat{j} + \hat{k}, and c⃗=4i^+16j^+k^\vec{c} = 4 \hat{i} + 16 \hat{j} + \hat{k} be three vectors parameterized by t∈Rt \in \mathbb{R}. Let p⃗=a⃗×b⃗\vec{p} = \vec{a} \times \vec{b}, q⃗=b⃗×c⃗\vec{q} = \vec{b} \times \vec{c}, and r⃗=c⃗×a⃗\vec{r} = \vec{c} \times \vec{a}. For t∈(0,6)t \in (0, 6) such that [p⃗ q⃗ r⃗]≠0[\vec{p} \, \vec{q} \, \vec{r}] \neq 0, let p⃗′,q⃗′,r⃗′\vec{p}', \vec{q}', \vec{r}' be the reciprocal system of vectors for p⃗,q⃗,r⃗\vec{p}, \vec{q}, \vec{r}. If f(t)f(t) is the continuous extension of the function g(t)=12[p⃗′ q⃗′ r⃗′]g(t) = \frac{1}{2 \sqrt{[\vec{p}' \, \vec{q}' \, \vec{r}']}} over the interval (0,6)(0, 6), find the sum of all values of tt at which f(t)f(t) attains a local extremum.

Options

  • A.

    3

  • B.

    9

  • C.

    6

  • D.

    12

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Bridge

Practice problems that bridge the gap between foundational theory and advanced practical application.

Concepts Behind This Question

First Derivative Test Trap|Vector Triple Product Expansion|Reciprocal System of Vectors

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to handle scalar triple products of reciprocal systems and the critical absolute value trap in vector magnitudes.

The trap is set by the square root of a squared scalar triple product, [a⃗b⃗c⃗]2\sqrt{[\vec{a}\vec{b}\vec{c}]^2}, which simplifies to ∣[a⃗b⃗c⃗]∣|[\vec{a}\vec{b}\vec{c}]|, not just [a⃗b⃗c⃗][\vec{a}\vec{b}\vec{c}].

Students who blindly apply the first derivative test to the algebraic polynomial without the absolute value will only find the local maximum at t=3t=3, missing the non-differentiable local minima at the roots t=2t=2 and t=4t=4 where the vectors become coplanar.

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Question Information

SubjectMathematics
ChapterApplication of Derivatives - Class 12
ConceptFirst Derivative Test Trap
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree