Proving angles are equal in a triangle in a circle.

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Discussion Overview

The discussion revolves around proving that angles in a triangle inscribed in a circle are equal, specifically focusing on the angles CAB and CBA. The scope includes theoretical reasoning and geometric properties related to circles and triangles.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that the proof may involve the exterior angle of a triangle being equal to the sum of the interior angles.
  • Another participant proposes using the inscribed angle theorem and its corollary about tangent lines to connect the angles to measures of arcs.
  • Multiple participants question how it can be established that arcs AC and CB are the same, indicating a need for clarification on this point.
  • A later reply introduces the Alternate Segment theorem, suggesting that the angle between the tangent and chord at the point of contact is equal to the angle in the alternate segment, which may relate to the angles in question.
  • One participant concludes that triangle CBA is isosceles based on the angles discussed.

Areas of Agreement / Disagreement

Participants express uncertainty about the equality of arcs AC and CB, and there is no consensus on the best approach to proving the angles are equal. The discussion includes competing ideas and methods without a clear resolution.

Contextual Notes

There are unresolved assumptions regarding the properties of the arcs and the application of theorems mentioned, which may affect the validity of the claims made.

markosheehan
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View attachment 6094

hi can someone help me work this out. i think it has something to do with the exterior angle of a triangle is equal to the sum of the interior angles but i can't work it.
 

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Prove that $\angle CAB=\angle CBA$ by connecting these angles to measures of arcs using the inscribed angle theorem and its corollary about tangent lines.
 
how do you know the arcs AC and CB are the same
 
Last edited:
markosheehan said:
how do you know the arcs AC and CB are the same

Are you familiar with the Alternate Segment theorem, which states that

The angle between the tangent and chord at the point of contact is equal to the angle in the alternate segment.

View attachment 6099

So now do you see a similarity between the angle marked in blue in the above diagram & the angle marked in blue in your diagram.

$\therefore$ It can be said that $\angle CAT = \angle CBA$ using alternate segment theorem

Now what can be said about the triangle CBA using it's angles?
 

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It's isosceles
 

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