Line l2 is 135°, not 90° as shown in your diagram. Why do you use a symmetrical diagram when an unsymmetrical diagram would immediately remove your confusion? Take 30° and 120°. tan 30° is y/x = 0.58.
tan 120° is x/-y = -1.73 . So, (y/x)*(x/-y)=-1 = 0.58*(-1.73) = -1 .