Infimum And Supremum In Triangles?

Geometry Level 5

Given a non-degenerate triangle A B C ABC with perimeter 16 with A 1 A_1 , B 1 B_1 and C 1 C_1 on B C BC , C A CA and A B AB such that A A 1 AA_1 , B B 1 BB_1 and C C 1 CC_1 are medians.

What is the sum of the infimum and supremum values of A A 1 + B B 1 + C C 1 AA_1+BB_1+CC_1 .


The answer is 28.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

In the above triangle, using simple triangle inequality considering triangles, AGB, BGC, AGC \text{AGB, BGC, AGC} we have,

{ 2 3 ( m a + m b ) > c 2 3 ( m a + m c ) > b 2 3 ( m c + m b ) > a \displaystyle \begin{cases} \frac{2}{3}(m_a+m_b) >c \\ \frac{2}{3}(m_a+m_c) >b \\ \frac{2}{3}(m_c+m_b) >a \end{cases}

Adding we get , m a + m b + m c > 3 4 ( a + b + c ) = 12 \displaystyle m_a+m_b+m_c > \frac{3}{4}(a+b+c)=12

Again by construction we claim that A D = D H AD=DH and since B D = D C BD=DC we have A B H C ABHC a parallelogram which says B H = A C BH=AC

So in Δ ABH \Delta \text{ABH} we have , 2 A D < A B + B H = A B + A C 2AD<AB+BH=AB+AC simillarly , 2 B E < B A + B C 2BE<BA+BC & 2 C F < A C + B C 2CF<AC+BC and adding we have ,

m a + m b + m c < ( a + b + c ) = 16 m_a+m_b+m_c<(a+b+c)=16 and thus 12 < A A 1 + B B 1 + C C 1 < 16 \displaystyle 12<AA_1+BB_1+CC_1<16 whch makes the answer 16 + 12 = 28 \boxed{16+12=28}

But to say these are not the max and min of the sum of medians as they would never attain this value whenever the triangle is non-degenerate. So in practical no such Extremum exists which it can attain, this is merely a upper and lower bound found till date which fits perfectly.

12 and 16 are attainable in the case of degenerate triangles, so all you need to from there is take the limit as a 'normal' triangle approximates a degenerate triangle.

For 16:

Consider the 'triangle' with side lengths 8 , 8 , 0 8,8,0

For 12:

Consider the 'triangle' with side lengths 4 , 4 , 8 4,4,8

So 12 and 16 are not attainable, but you can get arbitrarily close to these limits.

Wen Z - 4 years, 9 months ago

Log in to reply

wonderful Wen Z (+1), now the solution is complete and the problem and the answer is correct. That's what I wanted and was looking for. My apologies for Aditya (+1) and Sharky (+1). I'm going to delete my others comments...

Guillermo Templado - 4 years, 9 months ago

Perfect solution! +1

Sharky Kesa - 4 years, 9 months ago

Log in to reply

Another nice geometry problem of the day.

By the way, from which book you post these problems?

I don't have a good geometry book which has a lot of numerical problems to solve. Can you tell me other than GEOMETRY REVISITED ?

Priyanshu Mishra - 4 years, 9 months ago

Log in to reply

Mathematics Olympiad Treasures

Sharky Kesa - 4 years, 9 months ago

Log in to reply

@Sharky Kesa That also i have. I want the book which contains numerical problems , meaning to find something instead of proving.

Do you solve only proving questions in geometry from these books?

Priyanshu Mishra - 4 years, 9 months ago

Log in to reply

@Priyanshu Mishra Yeah, mainly those since I practice from them for maths olympiads.

Sharky Kesa - 4 years, 9 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...