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Ratio of Radii (Posted on 2008-03-27) Difficulty: 2 of 5
Let r, R, and s be the inradius, circumradius, and semiperimeter of triangle ABC.
If /A ≥ 90°, prove that

    r     a sin(A)
   --- ≤ ----------
    R        2s

  Submitted by Bractals    
Rating: 3.5000 (2 votes)
Solution: (Hide)
Look at triangle ABC. Since /A ≥ 90°, A must lie on or inside the circle with BC as a diameter.

Therefore, ha, the altitude on BC, is less than or equal to a/2.
       ha ≤ a/2

   ==> aha ≤ a2/2

   ==> 2rs ≤ a2/2

   ==> r ≤ [a/2s][a/2]

   ==> r ≤ [a/2s][R sin(A)]

        r     a sin(A)
   ==> --- ≤ ----------
        R        2s

Comments: ( You must be logged in to post comments.)
  Subject Author Date
SolutionA useful intermediate!Chesca Ciprian2008-03-27 16:32:22
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