if, from P, I wish to draw stright lines not meeting r, although I don't know if it will be possible because I don't know what happen at an infinite distance, I can use two different ways:
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When I present the idea of parallelism to my pupils I proceed in such a way:
if, from P, I wish to draw stright lines not meeting r, although I don't know if it will be possible because I don't know what happen at an infinite distance, I can use two different ways: |
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I say that usually we choose
QPR=90�
as angle of parallelism (euclidean
geometry); so doing we automatically esclude the second possibility and
parallelism is related only to angles created by PQ, QR and
PR and not to the distance from P and r. In such a way there is also one and only one parallel in P to r. |
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