Angle between two lines with same origin?

I need to calculate angle between two line segments with the same origin. One 开发者_开发问答line is perpendicular to the y-axis. Please suggest a method?

Let's say you have two lines. y=m1x+c1 and y=m2x+c1, lets say l1 and l2 respectively. If line l1 is perpendicular to the y axis, it means that l1 is parallel to x axis, so the slope of l1 is 0. So the angle between l1 and l2 will be arctan(m2).