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Question #74458
The Question
Let and be the roots of the equation , where . Consider the vectors , , and in . Let be the minimum value of such that there exists a real for which are coplanar for ALL real values of . Let be the minimum value of such that there exists a real for which are coplanar for EXACTLY ONE real value of . Find the value of .
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 : +5Common 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 () without considering as a valid state, completely missing the parameter range where coplanarity holds for all .
The problem forces the use of discriminant conditions and vertex formulas of quadratics to find the minimum values and .
Quick Facts About This Question
Question Information
| Subject | Mathematics |
| Chapter | Vector Algebra - Class 12 |
| Concept | Coplanarity Condition |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 170 sec |
| Access | Free |