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Measure that angle IV (Posted on 2004-11-02) Difficulty: 4 of 5
Triangle ABC is isosceles with AB=AC. Point D is on side AB such that angle BCD is 70 degrees. Point E is on side AC such that angle EBC is 60 degrees. Angle ABE equals 20 degrees, and angle DCE equals 10 degrees.

Find angle EDC. Justify your answer.

See The Solution Submitted by Brian Smith    
Rating: 3.6667 (9 votes)

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Solution Generalization | Comment 15 of 21 |
Let triangle ABC be isosceles with AB=AC. Angle BAC
equals 20 degrees. Let points D and E be on sides AB
and AC repectively such that angle BCD is greater than
angle CBE. What integer measures ( in degrees ) can
angles BCD and CBE take such that the measure ( in
degrees ) of angle EDC is an integer.
Let b, c, and d be the measures of angles CBE, BCD,
and EDC respectively. Let F be the intersection of
BE and CD. Applying the sine rule to triangles
  EDF:   EF*sin(b+c-d) = DF*sin(d)
  BDF:   BF*sin(80-b) = DF*sin(100-c)
  CEF:   CF*sin(80-c) = EF*sin(100-b)
  BCF:   BF*sin(b) = CF*sin(c)
Eliminating the lengths from these equations gives
     sin(d)       sin(b)*sin(80-c)*sin(100-c)     P
  ------------ = ----------------------------- = ---
   sin(b+c-d)     sin(c)*sin(80-b)*sin(100-b)     Q
solving for d we get
  tan(d) = ----------------
            Q + P*cos(b+c)
I wrote a small program in Perl and got
     50   20   10
     50   40   30
     60   30   10
     60   50   30   <--   Langley's Problem
     65   25    5
     65   60   40
     70   50   10
     70   60   20   <--   This Problem
There might be round off errors in the program, so
it would be nice to have a synthetic proof or
disproof for each of these cases.

  Posted by Bractals on 2004-11-06 14:28:55
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