Difference between revisions of "2010 IMO Problems/Problem 4"
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== Solution == | == Solution == | ||
=== Solution 1 === | === Solution 1 === | ||
− | [[Without loss of generality]], suppose that <math>AS > BS</math>. By Power of a Point, <math>SP^2 = SC^2 = SB \cdot SA</math>, so <math>\overline{SP}</math> is tangent to the circumcircle of <math>\triangle ABP</math>. Thus, <math>\angle KPS = 180 - \angle SPA = \angle ABP</math>. It follows that after some angle-chasing, <cmath>\begin{align*} | + | [[Without loss of generality]], suppose that <math>AS > BS</math>. By Power of a Point, <math>SP^2 = SC^2 = SB \cdot SA</math>, so <math>\overline{SP}</math> is tangent to the circumcircle of <math>\triangle ABP</math>. Thus, <math>\angle KPS = 180 - \angle SPA = \widehat{AP}/2 = \angle ABP</math>. It follows that after some angle-chasing, <cmath>\begin{align*} |
− | \widehat{ML} &= \widehat{MA} + 2\angle AKL = \left(2\angle CPK - \widehat{KC}\right) + 2\angle ABL \\ &= 2\left(\angle CPK + \angle KPS\right) - \widehat{KC} = 2\angle PCS - \widehat{KC} \\ &= \widehat{MK} | + | \widehat{ML} &= \widehat{MA} + 2\angle AKL \\ &= \left(2\angle CPK - \widehat{KC}\right) + 2\angle ABL \\ &= 2\left(\angle CPK + \angle KPS\right) - \widehat{KC} \\ &= 2\angle PCS - \widehat{KC} \\ &= \widehat{MK}, |
− | \end{align*} | + | \end{align*}</cmath> so <math>ML = MK</math> as desired. |
<asy> import graph; size(10.71cm); real lsf=0.5; pen dps=linewidth(0.7)+fontsize(9); defaultpen(dps); pen ds=black; real xmin=-2.22,xmax=4.48,ymin=-1.99,ymax=3; | <asy> import graph; size(10.71cm); real lsf=0.5; pen dps=linewidth(0.7)+fontsize(9); defaultpen(dps); pen ds=black; real xmin=-2.22,xmax=4.48,ymin=-1.99,ymax=3; | ||
pen ccqqqq=rgb(0.8,0,0), qqzzqq=rgb(0,0.6,0), evevff=rgb(0.9,0.9,1); | pen ccqqqq=rgb(0.8,0,0), qqzzqq=rgb(0,0.6,0), evevff=rgb(0.9,0.9,1); |
Revision as of 14:10, 11 July 2010
Problem
Let be a point interior to triangle
(with
). The lines
,
and
meet again its circumcircle
at
,
, respectively
. The tangent line at
to
meets the line
at
. Show that from
follows
.
Solution
Solution 1
Without loss of generality, suppose that . By Power of a Point,
, so
is tangent to the circumcircle of
. Thus,
. It follows that after some angle-chasing,
so
as desired.
Solution 2
See also
2010 IMO (Problems) • Resources | ||
Preceded by Problem 3 |
1 • 2 • 3 • 4 • 5 • 6 | Followed by Problem 5 |
All IMO Problems and Solutions |