Suppose we have two points. Both points are moving. Up to a certain time they are moving towards each other and afterwards they are moving away from each other. We need to find the minimum distance between the points and the the value of time, t, that gives this minimum distance.
r 1 (t)=3i+4j+t(5i-j) and r 2 (t)=5i-2j+t(i-3j)
In general the formula between two points in the
plane, 
is
If we write general points on r 1 and r 2 in coordinate form, remembering that I and j label the
and 
coordinates respectively, we get the points:
and 
And the distance between the points is
We now follow the usual procedure for completing the square.
Inside the square root, if we now put t=-0.4, the squared term is zero so the least distance is
