xpi0t0s's Avatar, Join Date: Aug 2004
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1
2 3
4 5 6
7 8 9 a
b c d e f
The only possible points below which a 4-unit triangle can exist are 1 2 3 4 5 6.
The only 4-unit triangle starting, e.g. at 7 is the one covering 7 4 8 2 5 9 which is the same as the one starting at 2.
xpi0t0s's Avatar, Join Date: Aug 2004
Mentor
Repost this time with code tags to preserve the formatting
Code:
    1
   2 3
  4 5 6
 7 8 9 a
b c d e f
Oops no there is another one, sorry, 456 89 d. 27 it is then :-)