why two lines joint eachother in inf?

Answer 1

I think you meant 'parallel' lines. And the reason they're said to join (or 'intersect') at infinity is because they map to the same point (the north pole) of a Riemann sphere centered at the origin.
Or, said another way, they intersect only at infinity because that's the limit value of their junctions distance from the origin as the angle between them approaces 0.

Doug

Answer 2

stand on a railway track which is straight and look at the tracks...
where they meet..
u will get your answer..

ps:be carefull of any oncoming trains......