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Three Dimensional Geometry

Question
CBSEENMA12033347

Determine whether the following pair of lines intersect:
      straight r with rightwards arrow on top space equals space straight i with hat on top space minus space straight j with hat on top space plus space straight lambda left parenthesis 2 space straight i with hat on top space plus space straight j with hat on top right parenthesis
and straight r with rightwards arrow on top space equals space 2 space straight i with hat on top space minus space straight j with hat on top space plus space straight mu left parenthesis straight i with hat on top space plus space straight j with hat on top space minus space straight k with hat on top right parenthesis

Solution
The equations of two lines are
               straight r with rightwards arrow on top space equals space straight i with hat on top space minus space straight j with hat on top space plus space straight lambda left parenthesis 2 space straight i with hat on top space plus space straight j with hat on top right parenthesis                   ...(1)
and       straight r with rightwards arrow on top space equals space 2 space straight i with hat on top space minus space straight j with hat on top space plus space straight mu left parenthesis straight i with hat on top space plus space straight j with hat on top space minus space straight k with hat on top right parenthesis             ...(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 stack straight b subscript 1 with rightwards arrow on top  and straight r with rightwards arrow on top space equals stack straight a subscript 2 with rightwards arrow on top plus space straight mu space stack straight b subscript 2 with rightwards arrow on top comma space space we space get comma
 stack straight a subscript 1 with rightwards arrow on top space equals space straight i with hat on top space minus space straight j 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 straight k with hat on top comma
stack straight a subscript 2 with rightwards arrow on top space equals space 2 space straight i with hat on top space minus space straight j with hat on top comma space space stack straight b subscript 2 with rightwards arrow on top space equals space straight i with hat on top space plus space straight j with hat on top space minus space straight k with hat on top
Let S be points on line (1) with position vector stack straight a subscript 1 with rightwards arrow on top and T be point on limit (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 straight i with hat on top
            stack straight b subscript 1 with rightwards arrow on top space cross times space 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 0 1 row 1 1 cell negative 1 end cell end table close vertical bar space equals space left parenthesis 0 minus 1 right parenthesis space straight i with hat on top space minus space left parenthesis negative 2 minus 1 right parenthesis space straight j with hat on top space plus space left parenthesis 2 minus 0 right parenthesis space straight k with hat on top space equals space minus straight i with hat on top space plus space 3 space straight j with hat on top space plus space 2 space straight k with hat on top
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 1 plus 9 plus 4 end root space equals space square root of 14
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 comma  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 space fraction numerator 1 over denominator square root of 14 end fraction left parenthesis negative straight i with hat on top space plus space 3 space straight j with hat on top space plus space 2 space straight k with hat on top right parenthesis
Now S.D. = Projection of ST with rightwards arrow on top space space on space 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 stack ST. with rightwards arrow on top space straight n with rightwards arrow on top
                 equals space straight i with hat on top. space fraction numerator 1 over denominator square root of 14 end fraction left parenthesis negative straight i with hat on top space plus space 3 space straight j with hat on top space plus space 2 space straight k with hat on top right parenthesis space equals space fraction numerator 1 over denominator square root of 14 end fraction left parenthesis negative 1 plus 0 plus 0 right parenthesis space equals space minus fraction numerator 1 over denominator square root of 14 end fraction
                  equals space fraction numerator 1 over denominator square root of 14 end fraction left parenthesis in space magnitude right parenthesis space not equal to 0
∴ given lines do not interesect.