## Sunday, July 8, 2012

### Problem 780: Acute Triangle, Sum of Inradius and Circumradius, Distances from the Orthocenter to the Vertices

Geometry Problem
Level: Mathematics Education, High School, Honors Geometry, College.

Click the figure below to see the complete problem 780.

1. Let D, E, F are the mid-points of BC, CA, AB respectively.

Then
1/2*a*OD + 1/2*b*OE + 1/2*c*OF
= Area of triangle ABC
= 1/2*(a+b+c)*r ....... (1)

Also, from OEAF, OFBD, ODCE concyclic, and by Ptolemy theorem,
R*a/2 = OF*b/2 + OE*c/2
R*b/2 = OF*a/2 + OD*c/2
R*c/2 = OE*a/2 + OD*b/2

Summing up,
1/2*(a+b+c)*R
= a/2*(OE+OF) + b/2*(OD+OF) + c/2*(OD+OE) ....... (2)

Now (1)+(2),
1/2*(a+b+c)*(R+r)
= 1/2*(a+b+c)*(OD+OE+OF)

Thus,
OD+OE+OF = R+r

On the other hand,
HA = 2*OD
HB = 2*OE
HC = 2*OF

Hence,
HA + HB + HC = 2*(R+r)

2. cos A + cos B + cos C = 1 + 4 sin A/2 . sin B/2 . sin C/2
=> 2R cos A + 2R cos B + 2R cos C = 2R + 2(4R sin A/2 . sin B/2 . sin C/2)
=> HA + HB + HC = 2R + 2r = 2(R + r)

3. It´s easy to express AH in terms of A, B, b as AH = b*cos(A)/sin(B), from extended Sine Law we can see that a/sin A = b/sin B = c/sin C = 2R, therefore AH = 2R*cos A, we gain BH and CH similarily, we obtain AH+BH+CH = 2R(cos(A)+cos(B)+cos(C)). The immediate consequence of Carnot´s theorem is that cos(A) + cos(B) + cos(C) = 1 + r/R. Using that we obtain the result.