MathematicsEquation of a Line in SpaceJEE Advanced 2006Moderate
Visualized Solution (English)
Problem Overview
- Part A: Calculate t=∑i=1∞tan−1(2i21) and find tant.
- Part B: Given sides a,b,c in AP, find tan2(2θ1)+tan2(2θ3).
- Part C: Find the perpendicular distance of a specific line from the origin.
Part A: Telescoping the Series
- General term: Ti=tan−1(2i21)
- Multiply numerator and denominator by 2: Ti=tan−1(4i22)
- Rewrite as: Ti=tan−1(1+(2i+1)(2i−1)(2i+1)−(2i−1))
Part A: Summing to Infinity
- Using identity: tan−1x−tan−1y=tan−1(1+xyx−y)
- Sum t=∑i=1∞[tan−1(2i+1)−tan−1(2i−1)]
- Limit as n→∞: t=tan−1(∞)−tan−1(1)=2π−4π=4π
- Therefore, tant=tan(4π)=1
Part B: Triangle and AP
- Sides a,b,c in AP ⟹2b=a+c
- Half-angle formula: tan2(2θ)=1+cosθ1−cosθ
- Given cosθ1=b+ca and cosθ3=a+bc
Part B: Substitution and Algebra
- tan2(2θ1)=1+b+ca1−b+ca=b+c+ab+c−a
- tan2(2θ3)=1+a+bc1−a+bc=a+b+ca+b−c
Part B: Final Result
- Sum =a+b+cb+c−a+a+b−c=a+b+c2b
- Since a+c=2b, denominator =2b+b=3b
- Sum =3b2b=32
Part C: 3D Geometry Setup
- Line passes through P(0,1,0) and is ⊥ to x+2y+2z=0.
- Direction of line = Normal to plane =(1,2,2).
- Equation of line: 1x=2y−1=2z=λ
Part C: Finding the Foot
- General point on line: Q(λ,2λ+1,2λ)
- Vector OQ=(λ,2λ+1,2λ)
- OQ⋅(1,2,2)=0⟹1(λ)+2(2λ+1)+2(2λ)=0
- Solve: 9λ+2=0⟹λ=−92
Part C: Calculating Distance
- Foot of perpendicular: Q(−92,95,−94)
- Distance OQ=(−92)2+(95)2+(−94)2
- OQ=814+25+16=8145=35
Final Matching
- Final Match:
- (A) → (p): tant=1
- (B) → (r): Sum =32
- (C) → (q): Distance =35
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