.
The no. Of straight lines obtained by joining 16 points on a plane, no three of them being on the same line is
~~~~~~~~~~~~~~~~~~
= = = 120.
= the number of combinations of 16 points taken 2 at a time.
On combinations, read the lessons
- Introduction to Combinations
- PROOF of the formula on the number of Combinations
- Problems on Combinations (*)
in this site.
By the way, in the lesson marked (*) you will find similar solved problem for planes in 3D . . .
Also, you have this free of charge online textbook in ALGEBRA-II in this site
- ALGEBRA-II - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Combinatorics: Combinations and permutations".