Is it true that one straight line goes through only 2 points??
If no then how ?? If yes then why??
No. A straight line goes through a lot of points. Infinitely many.
To add slightly to what @BvU said, it is true that there is only one straight line passing through a given pair of points. In Euclidean geometry, at least.
Yes, my reply was a bit corny. I understand english is not your native language and you meant something else than what the post litterally states. It shows that accurate communication isn't easy, to say the least.
In fact, this is one of the axioms (or postulates) of euclidean geometry. The first axiom, even. As such there is no proof or reason why - except that it's a reasonable thing to state. It is a starting point for a huge construct, namely geometry as we commonly know it (there are others, such as projective geometry).
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