(1) If you pass through three points on a plane (where any three points are not on the same straight line), drawing a straight line means that three points meet two. There is 3*(3- 1)/2 = 3.

If you pass through n points on a plane (where any three points are not on the same straight line), draw a straight line, that is, take the combination of two points of n points. There are N*(N- 1)/2 articles.

② The answer is C.2.3.4

In these four statements:

1. Because AM=MB, m is the midpoint of AB. If A.M.B is not on the same straight line, it is not established.

2. Take a point B on the extension line of line segment AM. If AB=2AM, then m is the midpoint of AB. Correct, because A.M.B is on the same straight line, MB=AB-AM=2AM-AM=AM, so M is the midpoint.

3. Because m is the midpoint of AB, AM=MB= 1/2AB. Correct, this is the nature of the midpoint.

4. because A.M.B is on the same straight line and AM=BM, m is the midpoint of AB. Correct, this is the definition of the midpoint.

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