How to calculate the angle between two lines (an urgent help needed)

Does anyone have any idea how to calculate two lines between four points. For example Line1= [x1,y1], [x2,y2] and Line2= [x3,y3], [x4,y4]. Then calculating the angle between these two lines.

I have found codes but they didn't work with me.

Thanks,

Fahad

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/29922-why-your-question-is-not-urgent-or-an-emergency

Answer 1

[Azzi Abdelmalek]

x1=2 ;y1=4;x2=3;y2=10
x3=1;y3=5;x4=6;y4=20
v1=(x2-x1)+j*(y2-y1)
v2=(x4-x3)+j*(y4-y3)
teta=angle(v2)-angle(v1)