Finding Angle between lines

i need to find an angle between lines

has in the image

whats i do

Consider some random vector points

[Image Analyst]

What do you have? Endpoint coordinates of each of the line segments? An image with all that stuff on there?

[OriAlpha]

ya i have both

[Image Analyst]

I doubt you have both. What created that image?

And I'm not going to do an image processing example if you have the endpoints in a vector and can do a simple math thing like the answer below (scroll down).

[OriAlpha]

below one is not clear

could you give me again

Answer 1

[zb li]

A(a,b) B(c,d) C(e,f)
AB = (c-a,d-b),BC = (e-c,f-d)
cosABC = AB*BC/|AB|*|BC| = (c-a,d-b)*(e-c,f-d)/|(c-a,d-b)|*|(e-c,f-d)|
so , acos(cosABC)*180/pi

[OriAlpha]

its not clear

could you tell me again

Answer 2

[Image Analyst]

See this link where it goes over it in extreme detail. Then use norm() and dot().