MathematicsScalar Triple ProductJEE Advanced 2009Moderate
Visualized Solution (English)
Introduction to the Problem
- Match Column-I (Expressions) with Column-II (Values).
- Part A: Trigonometric roots of 2sin2θ+sin22θ=2.
- Part B: Discontinuity of f(x)=[π6x]cos[π3x].
- Part C: Volume of a parallelepiped with given vector edges.
- Part D: Angle between unit vectors a and b given a+b+3c=0.
Part A: Simplifying the Equation
- Equation: 2sin2θ+sin22θ=2
- Apply identity: sin2θ=2sinθcosθ
- Substitute: 2sin2θ+(2sinθcosθ)2=2
- Simplify: 2sin2θ+4sin2θcos2θ=2
- Divide by 2: sin2θ+2sin2θcos2θ=1
Part A: Solving for θ
- Substitute cos2θ=1−sin2θ:
- sin2θ+2sin2θ(1−sin2θ)=1
- 2sin4θ−3sin2θ+1=0
- Factorize: (2sin2θ−1)(sin2θ−1)=0
- Roots: sin2θ=21⟹θ=π/4 (Matches q)
- Roots: sin2θ=1⟹θ=π/2 (Matches s)
Part B: Analyzing Discontinuity
- Function: f(x)=[π6x]cos[π3x]
- Discontinuity occurs when π6x∈Z or π3x∈Z.
- Check values from Column-II: π/6,π/4,π/3,π/2,π.
Part B: Testing Values
- At x=π/6: π6x=1 (Integer) ⟹ Discontinuous (p)
- At x=π/4: π6x=1.5,π3x=0.75 ⟹ Continuous
- At x=π/3: π6x=2,π3x=1 (Integers) ⟹ Discontinuous (r)
- At x=π/2: π6x=3 (Integer) ⟹ Discontinuous (s)
- At x=π: π6x=6,π3x=3 (Integers) ⟹ Discontinuous (t)
Part C: Volume of Parallelepiped
- Edges: u=i^+j^, v=i^+2j^, w=i^+j^+πk^
- Volume V=∣[uvw]∣=11112100π
- Expand along C3: V=∣π(2−1)∣=π
- Matches (t).
Part D: Angle Between Vectors
- Given: a+b+3c=0⟹a+b=−3c
- Square both sides: ∣a+b∣2=∣−3c∣2
- Expand: ∣a∣2+∣b∣2+2a⋅b=3∣c∣2
- Since unit vectors: 1+1+2cosθ=3(1)
- 2cosθ=1⟹cosθ=21⟹θ=π/3
- Matches (r).
Final Summary
- Final Matches:
- (A) → (q), (s)
- (B) → (p), (r), (s), (t)
- (C) → (t)
- (D) → (r)
- Key Takeaway: Multi-concept problems require systematic step-by-step analysis of each part independently.
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