14. Given (1) If is any point in the plane, prove:
(2) Let points be on sides respectively, and the interior angles of are respectively, prove: .
Solution
14. (1) In quadrilateral , applying the generalized Ptolemy's theorem (Ptolemy's inequality) we get
In , by the Law of Sines we have
which simplifies to
(2) From (1), we get
Adding the three inequalities, we get
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