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Three Dimensional Geometry

Question
CBSEENMA12033340

Find the shortest distance between the lines:
               straight r with rightwards arrow on top space equals space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 3 space straight k with hat on top space plus space straight lambda space open parentheses 2 space straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top close parentheses
and         straight r with rightwards arrow on top space equals space 2 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 5 space straight k with hat on top space plus space straight mu space open parentheses 3 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 4 space straight k with hat on top close parentheses.
      

Solution
The equations of the two lines are
                  straight r with rightwards arrow on top space equals space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 3 space straight k with hat on top space plus space straight lambda space open parentheses 2 space straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top close parentheses                       ...(1)
and            straight r with rightwards arrow on top space equals space 2 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 5 space straight k with hat on top space plus space straight mu space open parentheses 3 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 5 space straight k with hat on top close parentheses                    ...(2)
Comparing these equations with
       straight r with rightwards arrow on top space equals space stack straight a subscript 1 with rightwards arrow on top space plus space straight lambda space stack straight b subscript 1 with rightwards arrow on top space and space straight r with rightwards arrow on top space equals space stack straight a subscript 2 with rightwards arrow on top space plus space straight mu space stack straight b subscript 2 with rightwards arrow on top
We get,
        stack straight a subscript 1 with rightwards arrow on top space equals space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 3 space straight k with hat on top comma space space stack straight b subscript 1 with rightwards arrow on top space equals space 2 space straight i with hat on top space plus space 3 space straight j with hat on top space plus space 4 space straight k with hat on top
stack straight a subscript 2 with rightwards arrow on top space equals space 2 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 5 space straight k with hat on top comma space space stack straight b subscript 2 with rightwards arrow on top space equals space 3 space straight i with hat on top space plus space 4 space straight j with hat on top space plus space 5 space straight k with hat on top
Let S be point on line (1) with position vector stack straight a subscript 1 with rightwards arrow on top and T be point on line (2) with position vector stack straight a subscript 2 with rightwards arrow on top so that
   
                 ST with rightwards arrow on top space equals stack straight a subscript 2 with rightwards arrow on top space minus space stack straight a subscript 1 with rightwards arrow on top space equals space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 2 space straight k with hat on top
Now, stack straight b subscript 1 with rightwards arrow on top cross times stack straight b subscript 2 with rightwards arrow on top space equals space open vertical bar table row cell straight i with hat on top end cell cell straight j with hat on top end cell cell straight k with hat on top end cell row 2 3 4 row 3 4 5 end table close vertical bar
                      equals space left parenthesis 15 minus 16 right parenthesis space straight i with hat on top space minus space left parenthesis 10 minus 12 right parenthesis space straight j with hat on top space plus space left parenthesis 8 minus 9 right parenthesis space straight k with hat on top space equals negative straight i with hat on top space plus space 2 space straight j with hat on top space minus space straight k with hat on top
 therefore space space space open vertical bar stack straight b subscript 1 with rightwards arrow on top cross times stack straight b subscript 2 with rightwards arrow on top close vertical bar space equals space square root of left parenthesis negative 1 right parenthesis squared plus left parenthesis 2 right parenthesis squared plus left parenthesis negative 1 right parenthesis squared end root space equals square root of 6
Let PQ with rightwards arrow on top be the S.D. vector between given lines. Therefore, it is parallel to stack straight b subscript 1 with rightwards arrow on top cross times stack straight b subscript 2 with rightwards arrow on top.
If straight n with rightwards arrow on top is a unit vector along PQ with rightwards arrow on top
then
                    straight n with rightwards arrow on top space equals space fraction numerator stack straight b subscript 1 with rightwards arrow on top cross times stack straight b subscript 2 with rightwards arrow on top over denominator open vertical bar stack straight b subscript 1 with rightwards arrow on top cross times stack straight b subscript 2 with rightwards arrow on top close vertical bar end fraction space equals fraction numerator 1 over denominator square root of 6 end fraction left parenthesis negative straight i with hat on top space plus space 2 space straight j with hat on top space minus space straight k with hat on top right parenthesis
Now S.D. = Projection of ST with rightwards arrow on top space on space PQ with rightwards arrow on top space equals space Projection space of space ST with rightwards arrow on top space on space straight n with rightwards arrow on top space equals space ST with rightwards arrow on top. space straight n with rightwards arrow on top
                 equals space open parentheses straight i with hat on top space plus 2 space straight j with hat on top space plus space 2 space straight k with hat on top close parentheses space. space fraction numerator 1 over denominator square root of 6 end fraction space open parentheses negative straight i with hat on top space plus space 2 space straight j with hat on top space minus space straight k with hat on top close parentheses space equals space fraction numerator 1 over denominator square root of 6 end fraction left parenthesis negative 1 plus 4 minus 2 right parenthesis space equals space fraction numerator 1 over denominator square root of 6 end fraction
                     equals space left parenthesis 15 minus 16 right parenthesis space straight i with hat on top space space minus space left parenthesis 10 minus 12 right parenthesis space straight j with hat on top space plus space left parenthesis 8 minus 9 right parenthesis space straight k with hat on top space equals space minus straight i with hat on top space plus space 2 space straight j with hat on top space minus space straight k with hat on top

Some More Questions From Three Dimensional Geometry Chapter

Find the direction cosines of x, y and z-axis.