2016 USAJMO Problems/Problem 5
Problem
Let be an acute triangle, with
as its circumcenter. Point
is the foot of the perpendicular from
to line
, and points
and
are the feet of the perpendiculars from
to the lines
and
, respectively.
Given that prove that the points
and
are collinear.
Solution 1
It is well-known that (just use similar triangles or standard area formulas). Then by Power of a Point,
Consider the transformation
which dilates
from
by a factor of
and reflects about the
-angle bisector. Then
clearly lies on
, and its distance from
is
so
, hence we conclude that
are collinear, as desired.
Solution 2
We will use barycentric coordinates with respect to The given condition is equivalent to
Note that
Therefore, we must show that
Expanding, we must prove
Let such that
The left side is equal to
The right side is equal to
which is equivalent to the left hand side. Therefore, the determinant is
and
are collinear.
Solution 3
For convenience, let denote the lengths of segments
respectively, and let
denote the measures of
respectively. Let
denote the circumradius of
Clearly, Since
we have
Thus,
Note that Then, since
and
we have:
The Extended Law of Sines states that:
Since and
We see that
Rearranging we get:
We also have
so
by SAS similarity. Thus,
so
is a right angle.
Rearranging we get:
We also have
so
by SAS similarity. Thus,
so
is a right angle.
Since and
are both right angles, we get
so we conclude that
are collinear, so we are done. We also obtain the extra fact that
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.
See also
2016 USAJMO (Problems • Resources) | ||
Preceded by Problem 4 |
Followed by Problem 6 | |
1 • 2 • 3 • 4 • 5 • 6 | ||
All USAJMO Problems and Solutions |