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Three Dimensional Geometry

Question
CBSEENMA12033337

Find the shortest distance between the lines:
      straight r with rightwards arrow on top space equals space 3 space straight i with hat on top space plus space space 8 space straight j with hat on top space plus space 3 space straight k with hat on top space plus space straight lambda space left parenthesis 3 space straight i with hat on top space minus space straight j with hat on top space plus space straight k with hat on top right parenthesis
and straight r with rightwards arrow on top space equals space minus 3 space straight i with hat on top space minus space 7 space straight j with hat on top space plus space 6 space straight k with hat on top space plus space straight mu space open parentheses negative 3 space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 4 space straight k with hat on top close parentheses space.


Solution
The equations of the two lines are
          straight r with rightwards arrow on top space equals space 3 space straight i with hat on top space plus space space 8 space straight j with hat on top space plus space 3 space straight k with hat on top space plus space straight lambda space left parenthesis 3 space straight i with hat on top space minus space straight j with hat on top space plus space straight k with hat on top right parenthesis                        ...(1)
and    straight r with rightwards arrow on top space equals space minus 3 space straight i with hat on top space minus space 7 space straight j with hat on top space plus space 6 space straight k with hat on top space plus space straight mu space open parentheses negative 3 space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 4 space straight k with hat on top close parentheses space         ...(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 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 comma space we space get comma
stack straight a subscript 1 with rightwards arrow on top space equals space 3 space straight i with hat on top space plus space 8 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 3 space straight i with hat on top space minus space straight j with hat on top space plus space straight k with hat on top
stack straight a subscript 2 with rightwards arrow on top space equals space minus 3 space straight i with hat on top space minus space 7 space straight j with hat on top space plus space space 6 space straight k with hat on top comma space space stack straight b subscript 2 with rightwards arrow on top space equals space minus 3 space straight i with hat on top space plus space 2 space straight j with hat on top space plus space 4 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 space 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 minus 6 space straight i with hat on top space minus space 15 space straight j with hat on top space plus space 3 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 3 cell negative 1 end cell 1 row cell negative 3 end cell 2 4 end table close vertical bar
                            equals space left parenthesis negative 4 minus 2 right parenthesis space straight i with hat on top space minus space left parenthesis 12 plus 3 right parenthesis space straight j with hat on top space plus space left parenthesis 6 minus 3 right parenthesis space straight k with hat on top space equals space minus 6 space straight i with hat on top space minus space 15 space straight j with hat on top space plus space 3 space straight k with hat on top
therefore      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 36 plus 225 plus 9 end root space equals space square root of 270 space equals space 3 square root of 30
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 3 square root of 30 end fraction left parenthesis negative 6 space straight i with hat on top space minus space 15 space straight j with hat on top space plus space 3 space straight k with hat on top right parenthesis space equals space fraction numerator 1 over denominator square root of 30 end fraction left parenthesis negative space 2 space straight i with hat on top space minus space 5 space straight j with hat on top space plus 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 = Projection of ST with rightwards arrow on top space on space straight n with rightwards arrow on top space equals space stack ST. with rightwards arrow on top straight n with rightwards arrow on top
                 equals space open parentheses negative 6 space straight i with hat on top space minus space 15 space straight j with hat on top space plus space 3 space straight k with hat on top close parentheses space. space fraction numerator 1 over denominator square root of 30 end fraction left parenthesis negative 2 straight i with hat on top space minus space 5 space straight j with hat on top space plus space straight k with hat on top right parenthesis
space equals space fraction numerator 1 over denominator square root of 30 end fraction left parenthesis 12 plus 75 plus 3 right parenthesis space equals space fraction numerator 90 over denominator square root of 30 end fraction space equals space fraction numerator 3 cross times 30 over denominator square root of 30 end fraction space equals space 3 square root of 30 space units