7.3.1 Equation of a Line in Vector Form

How do I find the vector equation of a line?

• You need to know:
  • The position vector of one point on the line
  • A direction vector of the line (or the position vector of another point)

• There are two formulas for getting a vector equation of a line:
  • r = a + t (b - a)
    • use this formula when you know the position vectors a and b of two points on the line
  • r = a + t d
    • use this formula when you know the position vector a of a point on the line and a direction vector d
  • Both forms could be compared to the Cartesian equation of a 2D line
    • 
    • The point on the line a is similar to the “+c” part
    • The direction vector d or b – a is similar to the “m” part
• The vector equation of a line shown above can be applied equally well to vectors in 2 dimensions and to vectors in 3 dimensions
• Recall that vectors may be written using reference unit vectors or as column vectors 
• It follows that in a vector equation of a line either form can be employed – for example,

   and     

                             show the same equation written using the two different forms

How do I determine if a point is on a line?

• Each different point on the line corresponds to a different value of t
  • For example: if an equation for a line is r = 3i + 2j - k + t (i + 2j)
    • the point with coordinates (2, 0, -1) is on the line and corresponds to t = -1
  • However we know that the point with coordinates (-7, 5, 0) is not on this line
    • No value of t could make the k component 0

 

Can two different equations represent the same line?

• Why do we say a direction vector and not the direction vector? Because the magnitude of the vector doesn’t matter; only the direction is important
  • we can multiply any direction vector by a (non-zero) constant and this wouldn’t change the direction
• Therefore there are an infinite number of options for a (a point on the line) and an infinite number of options for the direction vector
• For Cartesian equations – two equations will represent the same line only if they are multiples of each other
  • and 
• For vector equations this is not true – two equations might look different but still represent the same line:
  • and