We have to find out which lines intersect the postive half axis of x.
According to this formula we can determine if the angle between two points(A[x1,y1] and B[x2,y2]) of the line (angle A0B where 0 is the center point of the coordinate system) is lower than 180 degree:
if $(y1<0)$ then if $x2*y1-x1*y2>0$ then the $angle < 180$
if $(y1 > 0)$ then if $x2*y1-x1*y2<0$ then the $angle < 180$
My question is how did we get this formula?