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=== Solution=== | === Solution=== | ||
Use mathematical induction. For <math>n=3</math> it is true because one point can't be closest to <math>P_3</math> in both ways, and that <math>1+2\le 3</math>. Suppose that for some <math>n</math>, the nearest adjacent points <math>P_a</math> and <math>P_b</math> on either side of <math>P_n</math> satisfy <math>a+b \le n</math>. Then consider the nearest adjacent points <math>P_c</math> and <math>P_d</math> on either side of <math>P_{n+1}</math>. It is by the assumption of the nearness we can see that either <math>(c,d)=(a+1,b+1)</math> still holds, or <math>P_1</math> jumps into the interior of the arc <math>P_{a+1}P_{n}P_{b+1}</math>, so that <math>c</math> or <math>d</math> equals two <math>1</math>. Let's consider the following two cases. | Use mathematical induction. For <math>n=3</math> it is true because one point can't be closest to <math>P_3</math> in both ways, and that <math>1+2\le 3</math>. Suppose that for some <math>n</math>, the nearest adjacent points <math>P_a</math> and <math>P_b</math> on either side of <math>P_n</math> satisfy <math>a+b \le n</math>. Then consider the nearest adjacent points <math>P_c</math> and <math>P_d</math> on either side of <math>P_{n+1}</math>. It is by the assumption of the nearness we can see that either <math>(c,d)=(a+1,b+1)</math> still holds, or <math>P_1</math> jumps into the interior of the arc <math>P_{a+1}P_{n}P_{b+1}</math>, so that <math>c</math> or <math>d</math> equals two <math>1</math>. Let's consider the following two cases. |
Revision as of 20:43, 6 May 2012
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Contents
Notes
USAJMO Problem 1
Given a triangle , let
and
be points on segments
and
, respectively, such that
. Let
and
be distinct points on segment
such that
lies between
and
,
, and
. Prove that
,
,
,
are concyclic (in other words, these four points lie on a circle).
Problem 2
Find all integers such that among any
positive real numbers
,
,
,
with
there exist three that are the side lengths of an acute triangle.
Problem 3
Let ,
,
be positive real numbers. Prove that
Problem 4
Let be an irrational number with
, and draw a circle in the plane whose circumference has length 1. Given any integer
, define a sequence of points
,
,
,
as follows. First select any point
on the circle, and for
define
as the point on the circle for which the length of arc
is
, when travelling counterclockwise around the circle from
to
. Supose that
and
are the nearest adjacent points on either side of
. Prove that
.
Solution
Use mathematical induction. For it is true because one point can't be closest to
in both ways, and that
. Suppose that for some
, the nearest adjacent points
and
on either side of
satisfy
. Then consider the nearest adjacent points
and
on either side of
. It is by the assumption of the nearness we can see that either
still holds, or
jumps into the interior of the arc
, so that
or
equals two
. Let's consider the following two cases.
(i) Suppose .
Since the length of the arc is
(where
equals to
subtracted by the greatest integer not exceeding
) and length of the arc
is
, we now consider a point
which is defined by
traveling clockwise on the circle such that the length of arc
is
. We claim that
is in the interior of the arc
. Algebraically, it is equivalent to either
or
.
Suppose the latter fails, i.e. . Then suppose
and
, where
,
are integers and
(
is not zero because
is irrational). We now have
and
Therefore is either closer to
than
on the
side, or closer to
than
on the
side. In other words,
is the closest adjacent point of
on the
side, or the closest adjacent point of
on the
side. Hence
or
is
, therefore
.
(ii) Suppose
Then either
when
and
, or
when one of
or
is
.
In either case, is true.
Problem 5
For distinct positive integers ,
, define
to be the number of integers
with
such that the remainder when
divided by 2012 is greater than that of
divided by 2012. Let
be the minimum value of
, where
and
range over all pairs of distinct positive integers less than 2012. Determine
.
Problem 6
Let be a point in the plane of triangle
, and
a line passing through
. Let
,
,
be the points where the reflections of lines
,
,
with respect to
intersect lines
,
,
, respectively. Prove that
,
,
are collinear.