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Question #73487

Difficulty: 8/10 | 🟢 Free

The Question

Let a⃗(t),b⃗(t),c⃗(t)\vec{a}(t), \vec{b}(t), \vec{c}(t) be three non-coplanar differentiable vector functions of t∈Rt \in \mathbb{R} such that their scalar triple product V(t)=[a⃗(t)b⃗(t)c⃗(t)]≥1V(t) = [\vec{a}(t) \vec{b}(t) \vec{c}(t)] \ge 1 for all t≥0t \ge 0, and V(0)=1V(0) = 1. Let a⃗′,b⃗′,c⃗′\vec{a}', \vec{b}', \vec{c}' be the reciprocal system of vectors corresponding to a⃗,b⃗,c⃗\vec{a}, \vec{b}, \vec{c}, and let W(t)=[a⃗′(t)b⃗′(t)c⃗′(t)]W(t) = [\vec{a}'(t) \vec{b}'(t) \vec{c}'(t)]. Define the functions: f(t)=da⃗dt⋅a⃗′+db⃗dt⋅b⃗′+dc⃗dt⋅c⃗′f(t) = \frac{d\vec{a}}{dt} \cdot \vec{a}' + \frac{d\vec{b}}{dt} \cdot \vec{b}' + \frac{d\vec{c}}{dt} \cdot \vec{c}' g(t)=[da⃗′dtb⃗′c⃗′]+[a⃗′db⃗′dtc⃗′]+[a⃗′b⃗′dc⃗′dt]g(t) = \left[ \frac{d\vec{a}'}{dt} \vec{b}' \vec{c}' \right] + \left[ \vec{a}' \frac{d\vec{b}'}{dt} \vec{c}' \right] + \left[ \vec{a}' \vec{b}' \frac{d\vec{c}'}{dt} \right] If f(t)f(t) and g(t)g(t) satisfy the relation f(t)+g(t)=2t3t2+1W(t)f(t) + g(t) = \frac{2t^3}{t^2+1} W(t) for all t≥0t \ge 0, find the value of the integral ∫02(f(t)+tW(t))dt\int_0^2 \left( f(t) + t W(t) \right) dt.

Options

  • A.

    32ln⁡5\frac{3}{2} \ln 5

  • B.

    ln⁡5\ln 5

  • C.

    52ln⁡3\frac{5}{2} \ln 3

  • D.

    3ln⁡23 \ln 2

Check Your Answer

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

Reciprocal System of Vectors|Vector Triple Product Expansion|Differentiation of Determinants

Why This Question Is Tricky

Examiner Levers

Recursive Logic

The answer to part A is required to solve part B (Step-wise failure test).

Difficulty Boost : +5

Common Mistakes

This question tests the student's ability to avoid brute-force expansion of vector triple products and instead use the properties of reciprocal systems combined with the differentiation of determinants.

The primary trap lies in attempting to differentiate the reciprocal vectors a⃗′=b⃗×c⃗V\vec{a}' = \frac{\vec{b} \times \vec{c}}{V} directly using the quotient rule, which leads to an insurmountable algebraic mess.

By recognizing that f(t)f(t) is the logarithmic derivative of the scalar triple product V(t)V(t) and g(t)g(t) is the derivative of its reciprocal W(t)W(t), the problem elegantly collapses into a non-linear differential equation.

A secondary trap involves solving V′−V′/V=…V' - V'/V = \dots, which requires recognizing the injectivity of the function x−ln⁡xx - \ln x for x≥1x \ge 1 to uniquely determine V(t)V(t).

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterVector Algebra - Class 12
ConceptReciprocal System of Vectors
Difficulty8
Cognitive Load7
Target Solve Time170 sec
AccessFree