angles between lines

[Joe_John]

I am trying to figure out a way to figure out the angle between to lines. These lines will always intersect. For example, if one line has the xy values of 100,0 and 100,200 and the other line has an xy value of 0,50 and 200,50. These two lines when ploted on a graph are 90 degrees from each other. I just cant figure out a way to do this. Any ideas?

[johnsold]

From the Analytic Geometry sections of some math handbooks:

The equation of a line passing through two points is: (y-y1)/(x-x1) = (y2-y1)/(x2-x1)

The general form of the equation for a straight line is: A*x + B*y + C = 0

The angle omega between two lines A1*x + B1*y +C1 = 0 and A2*x + B2*y + C2 = 0 is tan omega =  (A1*B2 - B1*A2)/(A1*A2 + B1*B2)

You can do the math to convert the equation in two-points form to the general form to get the angle.

Lynn 

[ben64]

That's basic linear algebra. Use the dot product of 2 vectors to find the cosinus of the angle between these vectors.

Ben

[Joe_John]

I'm still a little confused. do you have any exmples that might help?

[ben64 · Accepted solution]

Here is a way to do it. You can also use the math functions to do it as Lynn suggested.

Ben

[Joe_John]

I tried the code as you posted it but for some reason I thought I would be getting 90 degrees on the attached example.

[aeastet]

This is the long way to do it.

[Joe_John]

thanks here is my code modified to work

[altenbach]

I am aware that this thread is very old, but things gets much simpler if we use complex math. Try it!

(see also)

[billko]

---

@altenbach wrote:

I am aware that this thread is very old, but things gets much simpler if we use complex math. Try it!

---

So are you saying that the real solution is to use imaginary numbers?