Ptolemy's Inequality: Difference between revisions
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which is the desired inequality. Equality holds iff. <math>A </math>, <math>P </math>, and <math>{C} </math> are [[collinear]]. But since the angles <math>BAP </math> and <math>BDC </math> are congruent, this would imply that the angles <math>BAC </math> and <math> | which is the desired inequality. Equality holds iff. <math>A </math>, <math>P </math>, and <math>{C} </math> are [[collinear]]. But since the angles <math>BAP </math> and <math>BDC </math> are congruent, this would imply that the angles <math>BAC </math> and <math>BDC </math> are [[congruent]], i.e., that <math>ABCD </math> is a cyclic quadrilateral. | ||
==See Also== | ==See Also== | ||
Revision as of 20:39, 16 March 2009
Ptolemy's Inequality is a famous inequality attributed to the Greek mathematician Ptolemy.
Theorem
The inequality states that in for four points 
in the plane,
,
with equality if and only if 
is a cyclic quadrilateral with diagonals 
and 
.
Proof
We construct a point 
such that the triangles 
are similar and have the same orientation. In particular, this means that
.
But since this is a spiral similarity, we also know that the triangles 
are also similar, which implies that
.
Now, by the triangle inequality, we have 
. Multiplying both sides of the inequality by and using 
and 
gives us
,
which is the desired inequality. Equality holds iff. 
, , and 
are collinear. But since the angles 
and 
are congruent, this would imply that the angles 
and are congruent, i.e., that is a cyclic quadrilateral.
