Similar Posts

Subscribe
Notify of
3 Answers
Oldest
Newest Most Voted
Inline Feedbacks
View all comments
tunik123
4 months ago

(a) was helpful to see what this is all about 😉

So on with b)

D, E and F are the points of contact of the circle AB, BC or AC.

Angle DAM = angle MAF.

Angle MDA = angle AFM (= 90°).

Since the triangles DMA and MFA have the distance AM and also the incirculation radius r together, they are congruent.

This is followed by AF = AD. (I also know this as a tangent set.)

Also BE = BD.

AF + ME = AF + r = AD + r = a.

(AF and ME are parallel.)

And it is BE + MF = BE + r = BD + r = b.

The sum is AD + BD + 2r = a + b.

Because of AD + BD = c (hypotenuse)

is c + 2r = a + b.

c is at the same time the diameter of the circle, i.e.

2R + 2r = a + b

R + r = (a + b)/2, which was to be proven.

eterneladam
4 months ago
Reply to  tunik123

Again very elegant. I am not sure whether you have confused a and b in the meantime (AF + ME = b, BD + r = a), but has no consequences.

tunik123
4 months ago
Reply to  eterneladam

That can happen. But if I present the solution of Matheolympiaden tasks, the questioner should urgent check again. 😉