# Multivariable Calculus: Points in a Straight Line

1. Jun 1, 2014

### rmiller70015

Determine whether the points lie in a straight line:
1) A (2,4,2), B (3,7,-2), and C (1,3,3)
2) D (0,-5,5), E (1,-2,4), and F (3,4,2)

I'm not sure what method I need to use to show that they are or are not in a straight line. I know that the three points in a are not in a line but those in group b are in a line.

Last edited: Jun 1, 2014
2. Jun 1, 2014

### hilbert2

Suppose we have three points in 3d space, and their position vectors are $\vec{a},\vec{b}$ and $\vec{c}$. If the three points are on the same line, the difference vectors $\vec{b}-\vec{a}$ and $\vec{c}-\vec{b}$ have same (or opposite) direction and their cross product $(\vec{b}-\vec{a})\times (\vec{c}-\vec{b})$ is zero.

3. Jun 1, 2014

### rmiller70015

I get the cross product thing, but this is from a section be for we were introduced to dot and cross products, so I can't use them.

4. Jun 1, 2014

### Ray Vickson

If A,B,C are in a straight line, C-A must be a numerical (scalar) multiple of B-A. This works in any number of dimensions, not just three.

5. Jun 1, 2014

### rmiller70015

Should I use the distance formula to get the scalar then?

6. Jun 1, 2014

### Ray Vickson

Instead of asking, just start writing out the conditions that one (vector) difference is a scalar multiple of the other, and figure out if (a) this can possibly be true; and (2) if true, the value of the scalar.