294. The Octav Halunga's inequality in a triangle.

by Virgil Nicula, Jul 3, 2011, 1:10 PM

I found in one of the manuscripts of the my uncle Octav Halunga, teacher many years in the

high school Gh. Sincai - Bucuresti, the following nice and easy inequality in a triangle $ ABC$ :

$ \boxed{\boxed{\ ab+bc+ca\ge 4S\sqrt 3+\frac{1}{6}\left[(b+c-2a)^{2}+(c+a-2b)^{2}+(a+b-2c)^{2}\right]\ }}\ \ \ \mathrm{\ (Octav\ Halunga's\ inequality)\ }\ .$


Remark.The O.H.'s inequality is equivalently with the well-known inequality $ \boxed{\ p\sqrt 3\le 4R+r\ }$ and with the Hadwiger-Finsler's inequality

$ \boxed{\ \sum a^{2}\ge 4S\sqrt 3+\sum (b-c)^{2}\ }$ because $ \sum a^{2}-\sum (b-c)^{2}=$ $ \sum bc-\frac{1}{6}\sum (b+c-2a)^{2}=$ $ 2\sum bc-\sum a^{2}=$

$ 4\sum (p-b)(p-c)=$ $ 4r(4R+r)\ge 4S\sqrt 3$. From the O.H.'s inequality obtain the well-known inequality

$ a^{2}+b^{2}+c^{2}\ge \boxed{ab+bc+ca\ge 4S\sqrt 3}$ (See the my message from
here). Observe that $3\cdot\sum (b-c)^2=\sum (b+c-2a)^2$

and $a^2+b^2+c^2\ge\frac 13\cdot (a+b+c)^2\ge $ $ab+bc+ca\ge a\sqrt{bc}+b\sqrt {ca}+c\sqrt {ab}\ge $ $3\cdot\sqrt [3]{(abc)^2}\ge 4S\sqrt 3$ .

Remark. $12r(2R-r)\le a^2+b^2+c^2\le $ $4\left(2R^2+r^2\right)\ \mathrm{and}\ (b+c-2a)^2+(c+a-b)^2+(a+b-c)^2\le 8p\left(p-3r\sqrt 3\right)\ .$
This post has been edited 14 times. Last edited by Virgil Nicula, Nov 21, 2015, 12:19 PM

Comment

0 Comments

Own problems or extensions/generalizations of some problems which was posted here.

avatar

Virgil Nicula
Archives
+ October 2017
+ September 2017
+ December 2016
+ October 2016
+ February 2016
+ September 2013
+ October 2010
+ September 2010
Shouts
Submit
  • orzzzzzzzzz

    by mathMagicOPS, Jan 9, 2025, 3:40 AM

  • this css is sus

    by ihatemath123, Aug 14, 2024, 1:53 AM

  • 391345 views moment

    by ryanbear, May 9, 2023, 6:10 AM

  • We need virgil nicula to return to aops, this blog is top 10 all time.

    by OlympusHero, Sep 14, 2022, 4:44 AM

  • :omighty: blog

    by tigerzhang, Aug 1, 2021, 12:02 AM

  • Amazing blog.

    by OlympusHero, May 13, 2021, 10:23 PM

  • the visits tho

    by GoogleNebula, Apr 14, 2021, 5:25 AM

  • Bro this blog is ripped

    by samrocksnature, Apr 14, 2021, 5:16 AM

  • Holy- Darn this is good. shame it's inactive now

    by the_mathmagician, Jan 17, 2021, 7:43 PM

  • godly blog. opopop

    by OlympusHero, Dec 30, 2020, 6:08 PM

  • long blog

    by MrMustache, Nov 11, 2020, 4:52 PM

  • 372554 views!

    by mrmath0720, Sep 28, 2020, 1:11 AM

  • wow... i am lost.

    369302 views!

    -piphi

    by piphi, Jun 10, 2020, 11:44 PM

  • That was a lot! But, really good solutions and format! Nice blog!!!! :)

    by CSPAL, May 27, 2020, 4:17 PM

  • impressive :D
    awesome. 358,000 visits?????

    by OlympusHero, May 14, 2020, 8:43 PM

72 shouts
Tags
About Owner
  • Posts: 7054
  • Joined: Jun 22, 2005
Blog Stats
  • Blog created: Apr 20, 2010
  • Total entries: 456
  • Total visits: 404400
  • Total comments: 37
Search Blog
a