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Determine which points are on a line containing the point (-3,1,1) and parallel to the vector (1,-2,3)

Let P = (-3,1,1) be a point in \mathbb{R}^3 and let L be a line containing P and parallel to the vector (1,-2,3). Determine which of the following points are also on L.

  1. (0,0,0);
  2. (2,-1,4);
  3. (-2,-1,4);
  4. (-4,3,-2);
  5. (2,-9,16).

Given a point P= (-3,1,1) and a vector A = (1,-2,3) the line containing P in the direction of A is given by

    \[ L(P;A) = L((-3,1,1);(1,-2,3)) = \{ (-3,1,1) + t(1,-2,3) \mid t \in \mathbb{R} \} = \{(-3+t, 1-2t, 1+3t) \mid t \in \mathbb{R} \}. \]

  1. If (0,0,0) were on L then we must have some t \in \mathbb{R} such that

        \[ -3+t = 0, \qquad 1-2t = 0, \qquad 1+3t = 0. \]

    From the first equation, this requires t = 3, but then neither of the other two equations can hold. Hence, there is no t \in \mathbb{R} such that (0,0,0) = (-3+t,1-2t,1+3t) so (0,0,0) is not on the line.

  2. If (2,-1,4) were on L then we must have some t \in \mathbb{R} such that

        \[ -3+t = 2, \qquad 1-2t = -1, \qquad 1+3t = 4. \]

    From the first equation, this requires t = 5, but then neither of the other two equations can hold. Hence, there is no t \in \mathbb{R} such that (2,-1,4) = (-3+t,1-2t,1+3t) so (2,-1,4) is not on the line.

  3. If (-2,-1,4) is on L then we must have some t \in \mathbb{R} such that

        \[ -3+t = -2, \qquad 1-2t = -1, \qquad 1+3t = 4. \]

    From the first equation we have t = 1. This value of t also satisfies the other two equations. Hence, (-2,-1,4) = (-3+t, 1-2t, 1+3t) for t = 1. Therefore, (-2,-1,4) is on the line.

  4. If (-4,3,-2) is on L then we must have some t \in \mathbb{R} such that

        \[ -3+t = -4, \qquad 1-2t = 3, \qquad 1+3t = -2. \]

    From the first equation we have t = -1. This value of t also satisfies the other two equations. Hence, (-4,3,-2) = (-3+t, 1-2t, 1+3t) for t = -1. Therefore, (-4,3,-2) is on the line.

  5. If (2,-9,16) is on L then we must have some t \in \mathbb{R} such that

        \[ -3+t = 2, \qquad 1-2t = -9, \qquad 1+3t = 16. \]

    From the first equation we have t = 5. This value of t also satisfies the other two equations. Hence, (2,-9,16) = (-3+t, 1-2t, 1+3t) for t = 5. Therefore, (2,-9,16) is on the line.

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