Difference between revisions of "2009 AMC 12A Problems/Problem 20"
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===Note=== | ===Note=== | ||
− | The two triangles that are equal in area imply that <math>AB</math> is parallel to <math>DC</math> which implies that <math>\angle{EAB} = \angle{CDE}</math> and <math>\angle{EBA} = \angle{DCE}.</math> Furthermore, since <math>\angle{AEB} = \angle{DEC}</math> ( | + | The two triangles that are equal in area imply that <math>AB</math> is parallel to <math>DC</math> which implies that <math>\angle{EAB} = \angle{CDE}</math> and <math>\angle{EBA} = \angle{DCE}.</math> Furthermore, since <math>\angle{AEB} = \angle{DEC}</math> (vertical angles). By AAA similarity, <math>\triangle AEB \sim \triangle EDC.</math> |
~mathboy282 | ~mathboy282 |
Latest revision as of 15:56, 1 August 2024
- The following problem is from both the 2009 AMC 12A #20 and 2009 AMC 10A #23, so both problems redirect to this page.
Contents
Problem
Convex quadrilateral has
and
. Diagonals
and
intersect at
,
, and
and
have equal areas. What is
?
Solution 1
Let denote the area of triangle
.
, so
. Since triangles
and
share a base, they also have the same height and thus
and
with a ratio of
.
, so
.
![[asy]pathpen = linewidth(0.7);pointpen = black; pair D=MP("D",(0,0)),C=MP("C",(12,0)),A=MP("A",C+14*expi(145*pi/180),N),B=MP("B",A+(9,0),N),E=IP(A--C,B--D);MP("9",(A+B)/2,N);MP("12",(C+D)/2); fill(A--D--E--cycle,rgb(0.8,0.8,0.8));fill(B--C--E--cycle,rgb(0.8,0.8,0.8));D(A--B--C--D--cycle);D(A--C);D(B--D);D(E); [/asy]](http://latex.artofproblemsolving.com/d/2/5/d25c0c5968bc5ab64f9f91412801d2cdb764ce45.png)
Solution 2
Using the sine area formula on triangles and
, as
, we see that
![\[(AE)(ED) = (BE)(EC)\quad \Longrightarrow\quad \frac {AE}{EC} = \frac {BE}{ED}.\]](http://latex.artofproblemsolving.com/1/1/e/11e8d0633df9bf333912e3d0c13884bc95b2114e.png)
Since , triangles
and
are similar. Their ratio is
. Since
, we must have
, so
.
Solution 3(Fakesolve)
The easiest way for the areas of the triangles to be equal would be if they were congruent [1]. A way for that to work would be if were simply an isosceles trapezoid! Since
and
(look at the side lengths and you'll know why!),
Solution 4 (Easiest Way)
Using the fact that and the fact that
(which should be trivial given the two equal triangles) we have that
We know that so we have
Thus
But so we have
Simplifying gives
~mathboy282
Note
The two triangles that are equal in area imply that is parallel to
which implies that
and
Furthermore, since
(vertical angles). By AAA similarity,
~mathboy282
See also
2009 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 19 |
Followed by Problem 21 |
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 |
2009 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 |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.