2019 AMC 10B Problems/Problem 10
- The following problem is from both the 2019 AMC 10B #10 and 2019 AMC 12B #6, so both problems redirect to this page.
Contents
Problem
In a given plane, points and
are
units apart. How many points
are there in the plane such that the perimeter of
is
units and the area of
is
square units?
Solution 1
Notice that whatever point we pick for ,
will be the base of the triangle. Without loss of generality, let points
and
be
and
, since for any other combination of points, we can just rotate the plane to make them
and
under a new coordinate system. When we pick point
, we have to make sure that its
-coordinate is
, because that's the only way the area of the triangle can be
.
Now when the perimeter is minimized, by symmetry, we put in the middle, at
. We can easily see that
and
will both be
. The perimeter of this minimal triangle is
, which is larger than
. Since the minimum perimeter is greater than
, there is no triangle that satisfies the condition, giving us
.
~IronicNinja
Solution 2
Without loss of generality, let be a horizontal segment of length
. Now realize that
has to lie on one of the lines parallel to
and vertically
units away from it. But
is already 50, and this doesn't form a triangle. Otherwise, without loss of generality,
. Dropping altitude
, we have a right triangle
with hypotenuse
and leg
, which is clearly impossible, again giving the answer as
.
Solution 3
We have:
1. Area =
2. Perimeter =
3. Semiperimeter
We let:
1.
2.
3. .
Heron's formula states that for real numbers ,
,
, and semiperimeter
, the area is
.
Plugging numbers in, we have .
Square both sides, divide by and expand the polynomial to get
.
and the discriminant is
. Thus, there are no real solutions.
Solution 4 (graphing)
First, let's assume that A and B are and
respectively. The graph of "the perimeter is
" means that
. So this is the graph of an ellipse (memorize that!). Now let the endpoints of the major axis be
and
. Then
and
. So the
endpoints of the major axis are
and
. We can also figure out the endpoints of the minor axis must have a y-coordinate less than
. It is actually
.
Now, we consider "the area is ". Since the base has length
, then the height must have length
. So the graph of "the area is 100" is
lines, one at
and the other at
. However, this graph does NOT intersect the ellipse, as
. So, there are no intersections and thus no solutions, so the answer is
.
~Yrock
Video Solution
~Education, the Study of Everything
Video Solution
~IceMatrix
~savannahsolver
See Also
2019 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 9 |
Followed by Problem 11 | |
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 |
2019 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 5 |
Followed by Problem 7 |
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 |
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