2011 AIME I Problems/Problem 4
Contents
Problem
In triangle ,
,
and
. The angle bisector of angle
intersects
at point
, and the angle bisector of angle
intersects
at point
. Let
and
be the feet of the perpendiculars from
to
and
, respectively. Find
.
Solution 1
Extend and
such that they intersect line
at points
and
, respectively.
Since
is the angle bisector of angle
, and
is perpendicular to
, so
, and
is the midpoint of
. For the same reason,
, and
is the midpoint of
.
Hence
.
, so
.
Solution 2
Let be the incenter of
. Now, since
and
, we have
is a cyclic quadrilateral. Consequently,
. Since
, we have that
. Letting
be the point of contact of the incircle of
with side
, we have
. Thus,
Solution 3 (Bash)
Project onto
and
as
and
.
and
are both in-radii of
so we get right triangles with legs
(the in-radius length) and
. Since
is the hypotenuse for the 4 triangles (
and
),
are con-cyclic on a circle we shall denote as
which is also the circumcircle of
and
. To find
, we can use the Law of Cosines on
where
is the center of
. Now, the circumradius
can be found with Pythagorean Theorem with
or
:
. To find
, we can use the formula
and by Heron's,
. To find
, we can find
since
.
. Thus,
and since
, we have
. Plugging this into our Law of Cosines (LoC) formula gives
. To find
, we use LoC on
. Our formula now becomes
. After simplifying, we get
.
--lucasxia01
Solution 4
Because ,
is cyclic.
Ptolemy on CMIN:
by angle addition formula.
.
Let be where the incircle touches
, then
.
, for a final answer of
.
Video Solution
https://www.youtube.com/watch?v=vkniYGN45F4
~Shreyas S
See also
2011 AIME I (Problems • Answer Key • Resources) | ||
Preceded by Problem 3 |
Followed by Problem 5 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
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