Difference between revisions of "1991 IMO Problems/Problem 5"
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Revision as of 12:00, 12 November 2023
Problem
Let be a triangle and
an interior point of
. Show that at least one of the angles
is less than or equal to
.
Solution
Let ,
, and
be
,
,
, respcetively.
Let ,
, and
be
,
,
, respcetively.
Using law of sines on we get:
, therefore,
Using law of sines on we get:
, therefore,
Using law of sines on we get:
, therefore,
Multiply all three equations we get:
Using AM-GM we get:
Note that for ,
decreases with increasing
and fixed
Therefore, decreases with increasing
and fixed
From trigonometric identity:
,
since , then:
Therefore,
and also,
Adding these two inequalities we get:
Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.