Difference between revisions of "2001 AIME I Problems/Problem 9"
Line 32: | Line 32: | ||
By the barycentric area formula, our desired ratio is equal to | By the barycentric area formula, our desired ratio is equal to | ||
− | <cmath>\ | + | <cmath>\begin{align*} |
\begin{vmatrix} | \begin{vmatrix} | ||
1-p & p & 0 \ | 1-p & p & 0 \ | ||
0 & 1-q & q \ | 0 & 1-q & q \ | ||
r & 0 & 1-r \notag | r & 0 & 1-r \notag | ||
− | \end{vmatrix} =1-p-q-r+pq+qr+pr=1-(p+q+r)+\frac{(p+q+r)^2-( | + | \end{vmatrix} &=1-p-q-r+pq+qr+pr\ |
+ | &=1-(p+q+r)+\frac{(p+q+r)^2-(p^2+q^2+r^2)}{2}\ | ||
+ | &=1-\frac{2}{3}+\frac{\frac{4}{9}-\frac{2}{5}}{2}\ | ||
+ | &=\frac{16}{45} | ||
+ | \end{align*},</cmath> so the answer is <math>\boxed{61.}</math> | ||
== See also == | == See also == | ||
{{AIME box|year=2001|n=I|num-b=8|num-a=10}} | {{AIME box|year=2001|n=I|num-b=8|num-a=10}} |
Revision as of 21:05, 27 May 2016
Contents
[hide]Problem
In triangle ,
,
and
. Point
is on
,
is on
, and
is on
. Let
,
, and
, where
,
, and
are positive and satisfy
and
. The ratio of the area of triangle
to the area of triangle
can be written in the form
, where
and
are relatively prime positive integers. Find
.
Solution
Solution 1
![[asy] /* -- arbitrary values, I couldn't find nice values for pqr please replace if possible -- */ real p = 0.5, q = 0.1, r = 0.05; /* -- arbitrary values, I couldn't find nice values for pqr please replace if possible -- */ pointpen = black; pathpen = linewidth(0.7) + black; pair A=(0,0),B=(13,0),C=IP(CR(A,17),CR(B,15)), D=A+p*(B-A), E=B+q*(C-B), F=C+r*(A-C); D(D(MP("A",A))--D(MP("B",B))--D(MP("C",C,N))--cycle); D(D(MP("D",D))--D(MP("E",E,NE))--D(MP("F",F,NW))--cycle); [/asy]](http://latex.artofproblemsolving.com/a/a/5/aa532b18510960c694fe8213246465f219c7d942.png)
We let denote area; then the desired value is
![$\frac mn = \frac{[DEF]}{[ABC]} = \frac{[ABC] - [ADF] - [BDE] - [CEF]}{[ABC]}$](http://latex.artofproblemsolving.com/f/0/3/f031606bd0480fc8cb6ee9da5a70ec5dccdfcecb.png)
Using the formula for the area of a triangle , we find that
![$\frac{[ADF]}{[ABC]} = \frac{\frac 12 \cdot p \cdot AB \cdot (1-r) \cdot AC \cdot \sin \angle CAB}{\frac 12 \cdot AB \cdot AC \cdot \sin \angle CAB} = p(1-r)$](http://latex.artofproblemsolving.com/c/c/2/cc254e39ee583e6f0e7845a84cb9f1e473868472.png)
and similarly that and
. Thus, we wish to find
We know that
, and also that
. Substituting, the answer is
, and
.
Solution 2
By the barycentric area formula, our desired ratio is equal to
so the answer is
See also
2001 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 8 |
Followed by Problem 10 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.