# Thread: given 2 points, calculate the angle

1. If I were given two points (x,y) and (x',y'), how can I determine the angle they form? Let's assume that (x,y) is the vertex of the angle, and the other line the angle is to be measured from is (x+1,y). If the angle is to be defined by the area swept from (x+1,y), pivoting around (x,y) is a counterclockwise direction, and reaching the line connecting (x,y) and (x',y'), how would one calculate the angle?  2.

3. Originally Posted by gib65
If I were given two points (x,y) and (x',y'), how can I determine the angle they form? Let's assume that (x,y) is the vertex of the angle, and the other line the angle is to be measured from is (x+1,y). If the angle is to be defined by the area swept from (x+1,y), pivoting around (x,y) is a counterclockwise direction, and reaching the line connecting (x,y) and (x',y'), how would one calculate the angle?
Two points do not determine an angle, they determine a line.

If you mean the angle that is determined by the two points, plus the origin then that is quite a different thing. It might help to recognize that the dot product of two vectors is the product of their lengths and the cosine of the angle between them.  4. Originally Posted by Author
Two points do not determine an angle, they determine a line.
Well, why don't we add the point (x+1,y) like I said  Originally Posted by Author
It might help to recognize that the dot product of two vectors is the product of their lengths and the cosine of the angle between them.
Thank you kindly :-D  5. Originally Posted by gib65 Originally Posted by Author
Two points do not determine an angle, they determine a line.
Well, why don't we add the point (x+1,y) like I said Because that is not what you stated in the first sentence as being the basic problem. You additional note created quite a different problem, though in this case one that is well-defined and solvable by elementary means.

To do mathematics you need to pay attention to very strict logical rigor. That is a fundamental piece of the game.  Bookmarks
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