Prove Triangle Angles <= 30 Degrees | Let ABC & P Be a Triangle

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Discussion Overview

The discussion revolves around a geometric problem involving triangle ABC and a point P inside it. Participants are tasked with proving that at least one of the angles PAB, PBC, or PCA measures less than or equal to 30 degrees. The scope includes mathematical reasoning and proof techniques related to triangle properties.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant states that the sum of the angles Aº, Bº, and Cº in triangle ABC equals 180 degrees, suggesting that if the smallest angle is 60 degrees, then the angles PAB, PBC, or PCA must be less than or equal to 30 degrees.
  • Another participant questions the reasoning behind dividing 60 by 2 to arrive at 30 degrees.
  • Responses clarify that 60 degrees is the maximum value for the smallest angle in a triangle, implying that if any angle exceeds 60 degrees, the smallest angle must be less than 60 degrees.
  • A participant provides a visual reference to support their claim that angle PCA is less than 30 degrees in the case of an equilateral triangle.
  • Several participants express a need for a formal proof or logical demonstration of the claim.
  • References are made to the problem's appearance in the International Mathematical Olympiad (IMO) 1991, indicating its established context in mathematical competitions.

Areas of Agreement / Disagreement

Participants generally agree on the properties of triangle angles and the implications of the maximum smallest angle being 60 degrees. However, there is no consensus on a formal proof or demonstration of the claim regarding angles PAB, PBC, or PCA.

Contextual Notes

Some assumptions about the configuration of the triangle and the placement of point P remain unaddressed. The discussion does not resolve the mathematical steps needed to formally prove the claim.

bogdan
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Let ABC be a triangle and P a point inside it...
Prove that at least one of the angles PAB, PBC or PCA measures less or equal to 30 degrees...
 
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Aº+Bº+Cº = 180
Aº=Bº=Cº=60º is the largest value for the smallest angle.
(60º)/2=30º

If any of the angles (Aº,Bº or Cº) are greater than 60º then the smallest angle is less than 60º. Thus PAB, PBC or PCA measures less or equal to 30 degrees.
 
Why 60/2 ?
 
Because 60 is the greatest possible value that the smallest angle can be.

60+60+60=180 smallest 60
59+61+60=180 smallest 59
30+60+90=180 smallest 30

Its impossible for the smallest value to be greater than 60
 
Take a look...
http://www.angelfire.com/pro/fbi/tri.bmp
 
Last edited by a moderator:
PCA is less that 30 in that pic.
No matter where the point P is there will always be an angle equal to (in the case of an equalateral) or less than (in all other cases) 30.
 
Yeah...but I need a demonstration...:smile:
 
What do u mean by demonstration.
 
proof...logical...mathematical...
 
  • #10
Aº+Bº+Cº = 180
Aº=180-Bº-Cº where Aº is the smallest angle.
<PAC+<PAB = Aº
either <PAC or <PAB is less than or equal to 30
Is that mathematical enough
 
  • #11
another IMO question

This question appears in IMO 1991
 
  • #12
Of course...but it "looks" simple enough to be solved by anyone...
 
  • #13


Originally posted by KL Kam
This question appears in IMO 1991

would this be online and if so could you post the link?

is there a website with past IMO?
 
  • #14


Originally posted by marcus
would this be online and if so could you post the link?

is there a website with past IMO?

no need to post it, thanks anyway
I got it from google
 

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