Understanding displacements of points by interpreting directions

1. Nov 10, 2012

onako

Suppose that points x and y are given in Euclidean space. Point x is displaced to point x1 by

x1=x+a(x-y)

Given that a is positive number, how can it be shown that the distance x1 to y is larger than distance x to y. I'm mainly interested in a vector interpretation of the above update rule. In that sense, can (x-y) in the above rule be interpreted as
direction (force) from point y to point x? Similar interpretation is welcome.

Given negative a, the update is x1=x+a(y-x), so now the above intuition of a force is direction from point x to point y.

2. Nov 10, 2012

tiny-tim

hi onako!
yes, that's exactly correct

x-y is the vector $\vec{YX}$, with magnitude |YX| and direction from Y to X

3. Nov 12, 2012

onako

So, adding a(x-y) to x, means that the distance x to y changes depending on a: positive a implies increased distance, and negative a implies decreased distance? It's a bit "non-rigid" to state that a vector is added to a point.

4. Nov 12, 2012

tiny-tim

ah, no, it isn't added to a point, it's added to the vector $\vec{OX}$ …

$\vec{OX} + a\vec{XY} = \vec{OX_1}$

draw the triangle, and you'll see why!