MathematicsScalar (Dot) ProductJEE Advanced 2015Difficult
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Visualized Solution (English)

Multi-Concept Challenge Overview

  • The problem consists of four independent mathematical challenges.
  • Part A: Vector projections in .
  • Part B: Differentiability of piecewise functions.
  • Part C: Complex cube roots of unity and power series.
  • Part D: Harmonic Mean and Arithmetic Progressions.

Part A: Vector Projection Setup

  • Let and .
  • Magnitude of projection of on is .
  • Given: .
  • Simplifying the denominator: .

Part A: Solving for

  • Given relation: .
  • Substitute into :
  • .
  • or .
  • or . Thus, .

Part B: Continuity Condition

  • For to be differentiable at , it must be continuous at .
  • .
  • .
  • Equating them:

Part B: Differentiability and Solving

  • Differentiating: .
  • Equating derivatives at : .
  • Substitute (2) into (1): .
  • .

Part C: Complex Roots Insight

  • Let , , .
  • Observe: .
  • Also, and .
  • The equation becomes , where .

Part C: Power Condition for Zero

  • .
  • This holds if is not a multiple of .
  • .
  • So, .
  • For , possible values are .

Part D: Arithmetic Progression Relations

  • are in AP. Let common difference be .
  • .
  • .
  • .

Part D: Harmonic Mean Substitution

  • .
  • Substitute :
  • .
  • .

Part D: Final Calculation

  • Factorizing : .
  • Case 1: . Then . .
  • Case 2: . Then . .
  • Possible values for are and .

Final Matching and Conclusion

  • (A) Match: 2 (Option 1 in Column 2)
  • (B) Match: 1, 2 (Options 0, 1 in Column 2)
  • (C) Match: 1, 2, 4, 5 (Options 0, 1, 3, 4 in Column 2)
  • (D) Match: 2, 5 (Options 1, 4 in Column 2)

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