geometry
posted by Zack .
We are learning about proofs. We drew lines between noncollinear points. Our table showed 2 points >> 1 line segment, 3 point >> 3 line segments, 4 points >> 6 line segments, 5 points >> 10 line seg, 6 points >> 15 line seg. etc.
Write an expression in terms of n for the number of line segments that can be drawn between n points.
Please help.

1, 3, 6, 10,.. is the series
N*(N1)/2
where N is the number of points.
These are sometimes refered to as triangle numbers. (This of stacking pop cans in a triangle with rows of 1, 2, 3, 4 etc.)
See http://en.wikipedia.org/wiki/Triangular_numbers 
Thank you very much. I could explain it (I thought) but couldn't express it in math terms. Thanks again

line segment 6cm and divide 5 congruent parts

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