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If a line intersecting two other lines makes the alternate angles equal to one another, then the lines will be parallel.

Now if you go to this neat website


You see that the two "parallel" lines seem to converge eventually (it's off page). I am not sure if this is just "bad drawing" (doubt this) or the statement needs working.

What do they mean by 'alternate angle'? Exteror or Interior?

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Alternate here basically means on one side of the line then the other---alternating sides of the line. Hopefully that helps. –  Raymond Cheng Oct 18 '12 at 1:45
Do you have a picture? –  Hawk Oct 18 '12 at 2:28
They look perfectly parallel to me. Try measuring the distance between them at different points along their length. It might be an optical illusion. –  Alan Gee Oct 18 '12 at 7:59

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