MathematicsShortest Distance Between Two Skew LinesJEE Advanced 2008Moderate
Visualized Solution (English)
Identify Direction Vectors b1 and b2
- Given lines: L1:3x+1=1y+2=2z+1 and L2:1x−2=2y+2=3z−3
- Direction vector of L1: b1=3i^+j^+2k^
- Direction vector of L2: b2=i^+2j^+3k^
Calculate Cross Product b1×b2
- To find a vector n perpendicular to both L1 and L2, we use the cross product:
- n=b1×b2=i^31j^12k^23
- n=i^(3−4)−j^(9−2)+k^(6−1)=−i^−7j^+5k^
Find the Unit Vector n^
- Magnitude of n: ∣n∣=(−1)2+(−7)2+52=1+49+25=75=53
- The unit vector n^=∣n∣n=53−i^−7j^+5k^
- This matches option (b) for the first sub-question.
Identify Points A1 and A2 on the Lines
- Point on L1: A1(−1,−2,−1)⇒a1=−i^−2j^−k^
- Point on L2: A2(2,−2,3)⇒a2=2i^−2j^+3k^
Calculate Vector a2−a1
- Vector connecting points: a2−a1=(2−(−1))i^+(−2−(−2))j^+(3−(−1))k^
- a2−a1=3i^+0j^+4k^
Shortest Distance Calculation
- Shortest distance d=∣b1×b2∣∣(a2−a1)⋅(b1×b2)∣
- Numerator: ∣(3i^+4k^)⋅(−i^−7j^+5k^)∣=∣−3+0+20∣=17
- Denominator: 53
- d=5317. This matches option (d) for the second sub-question.
Equation of the Plane
- Plane passes through A1(−1,−2,−1) with normal n=−i^−7j^+5k^
- Equation: −1(x+1)−7(y+2)+5(z+1)=0
- −x−1−7y−14+5z+5=0⇒−x−7y+5z−10=0
- Simplified Equation: x+7y−5z+10=0
Distance of Point (1,1,1) from Plane
- Point P(1,1,1), Plane x+7y−5z+10=0
- Distance D=12+72+(−5)2∣1(1)+7(1)−5(1)+10∣
- D=1+49+25∣1+7−5+10∣=7513
- This matches option (c) for the third sub-question.
Summary and Key Takeaways
- Key Takeaways:
- The cross product b1×b2 provides the direction of the common perpendicular.
- Shortest distance is the projection of any vector connecting the lines onto the common perpendicular.
- The equation of a plane requires a point and a normal vector.
- Next Challenge: Prove that if the shortest distance is zero, the lines are coplanar.
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