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Question #74458

Difficulty: 8/10 | 🟢 Free

The Question

Let α\alpha and β\beta be the roots of the equation z2−2(t+2i)z+(k−4t+4it)=0z^2 - 2(t+2i)z + (k - 4t + 4it) = 0, where t,k∈Rt, k \in \mathbb{R}. Consider the vectors u⃗=αi^+βj^+αβk^\vec{u} = \alpha \hat{i} + \beta \hat{j} + \alpha\beta \hat{k}, v⃗=βi^+αj^+αβk^\vec{v} = \beta \hat{i} + \alpha \hat{j} + \alpha\beta \hat{k}, and w⃗=i^+j^+ck^\vec{w} = \hat{i} + \hat{j} + c \hat{k} in C3\mathbb{C}^3. Let S1S_1 be the minimum value of kk such that there exists a real tt for which u⃗,v⃗,w⃗\vec{u}, \vec{v}, \vec{w} are coplanar for ALL real values of cc. Let S2S_2 be the minimum value of kk such that there exists a real tt for which u⃗,v⃗,w⃗\vec{u}, \vec{v}, \vec{w} are coplanar for EXACTLY ONE real value of cc. Find the value of S12+S22S_1^2 + S_2^2.

Options

  • A.

    68

  • B.

    72

  • C.

    64

  • D.

    80

Check Your Answer

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

Coplanarity Condition|Collinearity and Coplanarity|Conjugate Root Theorem|Maximum/Minimum of Quadratic Expressions

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 intersection of coplanarity, complex quadratics, and parameter optimization.

The primary trap lies in the 'Conjugate Root Theorem': students often instinctively assume that roots of a quadratic are complex conjugates, leading them to incorrectly force the imaginary part of the sum of roots to zero.

However, since the coefficients are complex, the roots are not conjugates.

Furthermore, students frequently divide by (α−β\alpha-\beta) without considering α=β\alpha=\beta as a valid state, completely missing the parameter range S1S_1 where coplanarity holds for all cc.

The problem forces the use of discriminant conditions and vertex formulas of quadratics to find the minimum values S1S_1 and S2S_2.

Quick Facts About This Question

Question Information

SubjectMathematics
ChapterVector Algebra - Class 12
ConceptCoplanarity Condition
Difficulty8
Cognitive Load7
Target Solve Time170 sec
AccessFree