2017 AMC 10A Problems/Problem 23
Problem
How many triangles with positive area have all their vertices at points 
in the coordinate plane, where 
and 
are integers between 
and 
, inclusive?
Solution
We can solve this by finding all the combinations, then subtracting the ones that are on the same line. There are 
points in all, from 
to 
, so 
is 
, which simplifies to 
.
Now we count the ones that are on the same line. We see that any three points chosen from and 
would be on the same line, so 
is 
, and there are rows, columns, and 
long diagonals, so that results in 
.
We can also count the ones with 
on a diagonal. That is 
, which is 4, and there are of those diagonals, so that results in 
.
We can count the ones with only 
on a diagonal, and there are diagonals like that, so that results in .
We can also count the ones with a slope of 
, , 
, or 
, with points in each. There are 
of them, so that results in .
Finally, we subtract all the ones in a line from , so we have 
See Also
| 2017 AMC 10A (Problems • Answer Key • Resources) | ||
| Preceded by Problem 22 |
Followed by Problem 24 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
| All AMC 10 Problems and Solutions | ||
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