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Homework Help: Calc 2 Lines and planes 1

  1. Apr 29, 2010 #1
    1. The problem statement, all variables and given/known data

    Find the vector and parametric equations for the line through the point P(-3, 3, -5) and the point Q(-7, 4, -1).

    2. Relevant equations

    N/A

    3. The attempt at a solution

    r= <__,__,-5>+t<__,__,4> The -5 and 4 are given.

    Heres what I have so far. r=<-3,3-5>+t<__,__,__>

    I am totally lost on how to find those other two points and the equations.
     
  2. jcsd
  3. Apr 29, 2010 #2

    Mark44

    Staff: Mentor

    Think of it in terms of adding vectors - one vector from the origin to a given point on the line, and another vector from the given point on the line to an arbitrary point on the line.

    Your vector equation of the line will be r = OP + t*PQ

    What you're missing is a vector that gives the direction of the line.
     
  4. Apr 29, 2010 #3
    Ok, Is the value of 4, which is givin in the problem as the z value in the second vector, a clue at all?

    I tried multiplying P and Q and then dropping them all down so that they agree with the z=4 value and that didnt work. Its not that simple is it? PQ doesnt mean I can just multiply P and Q together does it?
     
  5. Apr 29, 2010 #4

    Mark44

    Staff: Mentor

    How do you find the vector from P to Q? You don't multiply - that makes no sense at all.
     
  6. Apr 29, 2010 #5
    Oh ok, Im still thinking in basic algebra. Not much time spent on vectors yet.

    So I would to Q-P to get PQ.
     
  7. Apr 30, 2010 #6

    Mark44

    Staff: Mentor

    Yes.
     
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