2021 AIME II Problems/Problem 12
Contents
Problem
A convex quadrilateral has area and side lengths
and
in that order. Denote by
the measure of the acute angle formed by the diagonals of the quadrilateral. Then
can be written in the form
, where
and
are relatively prime positive integers. Find
.
Diagram
~MRENTHUSIASM (by Geometry Expressions)
Solution 1
We denote by ,
,
and
four vertices of this quadrilateral, such that
,
,
,
.
We denote by
the point that two diagonals
and
meet at.
To simplify the notation, we denote
,
,
,
.
We denote .
Hence,
and
.
First, we use the triangle area formula with sines to write down an equation of the area of the quadrilateral .
We have
where the second equality follows from the formula to use the sine function to compute a triangle area, the the fourth equality follows from the property that
.
Because , we have
.
Second, we use the law of cosines to establish four equations for four sides of the quadrilateral .
In , following from the law of cosines, we have
Because and
, we have
In , following from the law of cosines, we have
Because and
, we have
In , following from the law of cosines, we have
Because and
, we have
In , following from the law of cosines, we have
Because and
, we have
By taking , we get
By taking , we get
Therefore, by writing this answer in the form of , we have
and
.
Therefore, the answer to this question is
.
~ Steven Chen (www.professorchenedu.com)
Solution 2
Since we are asked to find , we can find
and
separately and use their values to get
. We can start by drawing a diagram. Let the vertices of the quadrilateral be
,
,
, and
. Let
,
,
, and
. Let
,
,
, and
. We know that
is the acute angle formed between the intersection of the diagonals
and
.
We are given that the area of quadrilateral is
. We can express this area using the areas of triangles
,
,
, and
. Since we want to find
and
, we can represent these areas using
as follows:
We know that . Therefore it follows that:
From here we see that . Now we need to find
. Using the Law of Cosines on each of the four smaller triangles, we get following equations:
We know that . We can substitute this value into our equations to get:
If we subtract the sum of the first and third equation from the sum of the second and fourth equation, the squared terms cancel, leaving us with:
From here we see that .
Since we have figured out and
, we can calculate
:
Therefore our answer is .
~ my_aops_lessons
Solution 3 (Pythagorean Theorem and Right Triangle Trigonometry)
This solution refers to the Diagram section.
In convex quadrilateral let
and
Let
and
be the feet of the perpendiculars from
and
respectively, to
We obtain the following diagram:
Let and
Applying the Pythagorean Theorem to right triangles
and
we respectively get
Let the brackets denote areas. We get
We subtract
from
From right triangles
and
we have
It follows that
Finally, dividing
by
gives
from which the answer is
~MRENTHUSIASM
Video Solution
https://www.youtube.com/watch?v=7DxIdTLNbo0
See also
2021 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 11 |
Followed by Problem 13 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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