Difference between revisions of "2011 AIME I Problems/Problem 4"
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== Solution 1 == | == Solution 1 == | ||
− | Extend <math>{CM}</math> and <math>{CN}</math> such that they | + | Extend <math>{CM}</math> and <math>{CN}</math> such that they intersect lines <math>{AB}</math> at points <math>P</math> and <math>Q</math>, respectively. |
− | Since <math>{BM}</math> is the angle bisector of angle B,and <math>{CM}</math> is perpendicular to <math>{BM}</math> ,so , <math>BP=BC=120</math>, M is the midpoint of <math>{CP}</math> .For the same reason,<math>AQ=AC=117</math>,N is the midpoint of <math>{CQ}</math>. | + | Since <math>{BM}</math> is the angle bisector of angle B,and <math>{CM}</math> is perpendicular to <math>{BM}</math> ,so , <math>BP=BC=120</math>, M is the midpoint of <math>{CP}</math>. For the same reason, <math>AQ=AC=117</math>,N is the midpoint of <math>{CQ}</math>. |
− | Hence<math>MN=\frac{PQ}{2}</math>.But <math>PQ=BP+AQ-AB=120+117-125=112</math>,so<math>MN=\boxed{56}</math>. | + | Hence <math>MN=\frac{PQ}{2}</math>. But <math>PQ=BP+AQ-AB=120+117-125=112</math>,so <math>MN=\boxed{56}</math>. |
== Solution 2 == | == Solution 2 == |
Revision as of 22:47, 9 August 2018
Problem 4
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 lines
at points
and
, respectively.
Since
is the angle bisector of angle B,and
is perpendicular to
,so ,
, M is the midpoint of
. For the same reason,
,N is the midpoint of
.
Hence
. But
,so
.
Solution 2
[There seem to be some mislabeled points going on here but the idea is sound.]
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 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
.
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 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.