how to find out the angle?

Hi All,
This might be a math question. I want to find out the angle between two lines which have the same length.


Line one : AB A(Xa,Ya) B(Xb,Yb)
Line two : CD C(Xc,Yc) D(Xd,Yd)

move A to C , so B =
new_x = Xb - ( Xa-Xc )
new_y = Yb - ( Ya-Yc )

the find out the length of new_B and D
dist = sqrt ( (new_x - Xd)* (new_x - Xd) + (new_y - Yd)* (new_y - Yd) );

finally, the angle = 2*( asin( dist/2/line_AB );



=================================

use:
arc = degree*PI/180;
sin( arc ).....
cos( arc )....



this is what I have done so far, but it cannot show the corret angle.
anyone can tell me what's wrong above??

If both lines are the same length, then you can't use normal trig functions (sin, asin, etc) to figure out their angle because in a rt triangle, the only way to have two lines of equal length is to have the 90 degree angle right between them....

my idea is to divide the triangle (Isosceles) to two right triangles.
then find out the angle in one of these two right trigangle, and mutiple *2 is the answer