2004 AMC 12A Problems/Problem 13
Problem
Let be the set of points
in the coordinate plane, where each of
and
may be
,
, or
. How many distinct lines pass through at least two members of
?
Solution
Solution 1
Let's count them by cases:
- Case 1: The line is horizontal or vertical, clearly
.
- Case 2: The line has slope
, with
through
and
additional ones one unit above or below those. These total
.
- Case 3: The only remaining lines pass through two points, a vertex and a non-vertex point on the opposite side. Thus we have each vertex pairing up with two points on the two opposites sides, giving
lines.
These add up to .
Solution 2
There are ways to pick two points, but we've clearly overcounted all of the lines which pass through three points. In fact, each line which passes through three points will have been counted
times, so we have to subtract
for each of these lines. Quick counting yields
horizontal,
vertical, and
diagonal lines, so the answer is
distinct lines.
Solution 3
First consider how many lines go through and hit two points in
. You can see that there are
such lines. Now, we cross out
and make sure to never consider consider lines that go through it anymore (As doing so would be double counting). Repeat for
, making sure not to count the vertical line as it goes through the crossed out
. Then cross out
. Repeat for the rest, and count
lines in total.
See Also
2004 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 12 |
Followed by Problem 14 |
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 12 Problems and Solutions |