I have two lines, the angle between them is 'a'.
Will the lines become parallel If I rotate one of them at the angle 'a' around a pivot point P(x,y)?
I guess the answer is yes.
if the lines are in the same plane then its very easy to figure out.
but if there are skew lines then the angle should be the angle between there projection.for figuring out projection take it on such a plane in which projection obtained is a pair of lines.after obtaining the angle even rotation should be proper i.e its not like that one is rotating but the line instead becoming parallel is cutting the other line.