2009 USAMO Problems/Problem 1
Contents
Problem
Given circles and
intersecting at points
and
, let
be a line through the center of
intersecting
at points
and
and let
be a line through the center of
intersecting
at points
and
. Prove that if
and
lie on a circle then the center of this circle lies on line
.
Solution
Let be the circumcircle of
,
to be the radius of
, and
to be the center of the circle
, where
. Note that
and
are the radical axises of
,
and
,
respectively. Hence, by power of a point(the power of
can be expressed using circle
and
and the power of
can be expressed using circle
and
),
Subtracting these two equations yields that
, so
must lie on the radical axis of
,
.
Remarks
Conveniently, this analytic solution takes care of all configuration issues we may have encountered had we used a more traditional solution, such as angle-chasing(which would indeed work).
~AopsUser101
See also
2009 USAMO (Problems • Resources) | ||
Preceded by First question |
Followed by Problem 2 | |
1 • 2 • 3 • 4 • 5 • 6 | ||
All USAMO Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.