MHB Does the Triangle's Centroid Lie Inside the Circle with Diameter OA?

  • Thread starter Thread starter anemone
  • Start date Start date
  • Tags Tags
    2017
AI Thread Summary
In the discussion, participants are tasked with proving that the centroid G of triangle ABC lies within the circle defined by diameter OA, given that side BC is the longest. The problem emphasizes the relationship between the triangle's centroid and circumcenter, particularly under the condition of BC being the largest side. The thread also notes a lack of responses to the previous week's problem, indicating a need for engagement. A suggested solution is available for those seeking guidance. The discussion highlights the importance of understanding geometric properties in triangle configurations.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Here is this week's POTW:

-----

Let $ABC$ be a triangle with centroid $G$ and circumcenter $O$. Prove that if $BC$ is its largest side, then $G$ lies in the interior of the circle with diameter $OA$.

-----

Remember to read the http://www.mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to http://www.mathhelpboards.com/forms.php?do=form&fid=2!
 
Physics news on Phys.org
No one answered last week's problem.(Sadface)

You can find the suggested solution as follows:
View attachment 6469
Let $BC=a,\,CA=b,\,AB=c$, $M$ be the midpoint of $BC$ and $D$ be the second intersection point between $AM$ and the circumcircle of triangle $ABC$.

By the Power of A Point theorem, we have $AM\cdot MD=BM \cdot MC$. Thus we get

$\begin{align*}AD&=AM+MD\\&=m_a+\dfrac{\dfrac{a}{2}\cdot \dfrac{a}{2}}{m_a}\\&=\dfrac{4m_a^2+a^2}{4m_a}\\&=\dfrac{b^2+c^2}{2m_a}\end{align*}$

Now, taking into consideration that the circle of diameter $OA$ is the locus of midpoints of chords of $O$ that pass through $A$, we have $G$ lies in the interior of the circle of diameter $OA$ if and only if $AG<\dfrac{AD}{2}$, or equivalently

$\dfrac{2}{3}m_a<\dfrac{1}{2}\left(\dfrac{b^2+c^2}{2m_a}\right)$

$8m_a^2<3(b^2+c^2)$

$b^2+c^2<2a^2$

Since $BC$ is the greatest side of triangle $ABC$, the last inequality holds so we are done.
 

Attachments

  • POTW255.png
    POTW255.png
    10.2 KB · Views: 117
Back
Top