Inequality with Circle and Triangle in Euclidean Geometry

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SUMMARY

The discussion focuses on the geometric relationship between circles and triangles, specifically addressing the implications of angles in Euclidean geometry. The user references Euclid's propositions, particularly I.32 and I.19, to establish the relationships between angles and sides in the context of a triangle inscribed in a circle. The conclusion drawn is that the angle relationships lead to definitive inequalities between the lengths of the segments OA and OD. This analysis is crucial for understanding the foundational principles of Euclidean geometry.

PREREQUISITES
  • Understanding of Euclidean geometry principles
  • Familiarity with Euclid's propositions, specifically I.32 and I.19
  • Basic knowledge of angle relationships in triangles and circles
  • Ability to interpret geometric diagrams and proofs
NEXT STEPS
  • Study Euclid's Elements, focusing on propositions I.32 and I.19
  • Explore the implications of angle inequalities in triangle geometry
  • Learn about inscribed angles and their properties in circles
  • Investigate advanced geometric proofs involving circles and triangles
USEFUL FOR

Students of geometry, educators teaching Euclidean principles, and anyone interested in the foundational aspects of geometric proofs and inequalities.

seniorhs9
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Homework Statement



Please see below...

Homework Equations



Please see below...

The Attempt at a Solution



Hi. This question is on geometry with circle and triangle. I am stuck only on 2 parts of the solution and not the whole solution...

Thank you...

http://img256.imageshack.us/img256/9475/gtewp249no24.jpg

[ How did they get this? They never explained or proved this and it is NOT obvious from the picture.

Blue: How is that implication true? By what theorem or reasoning?
 
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To see that the red statement is true, observe that \angleADO = \angleCDB > \angleBOD by Euclid I.32 (book I proposition 32).

In fact, that statement is superfluous. Once you know that \angleADO > \angleBDO = \angleADC = \angleAOD + \angleOAD, you know that \angleADO > \angleOAD, and by Euclid I.19, OA > OD.
 

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