246. R1-R2=OI (nice).

by Virgil Nicula, Mar 6, 2011, 5:03 PM

PP. Let $\triangle ABC$ and $d$ for which $A\in d$ , $d\perp IO$ and meet $BC$ at $K$ . Denote the lengths $R_1$ , $R_2$ of the circumradii for $\triangle AKB$ , $\triangle AKC$ . Prove that $OI=R_1-R_2$ .

A preliminary study. Suppose $K\in (BC)\ ,\ AK\perp OI$ and $b\ne c\ .$ Denote $\alpha =m(\widehat {BAK})$ and $\beta =m(\widehat {CAK})\ .$ Using an elementary metrics we obtain :

$1.\blacktriangleright \sum a(a-b)(a-c)=4pr(R-2r)\ \ \ ;\ \ \ 2.\blacktriangleright\frac{KB}{c(b-a)}=$ $\frac{KC}{b(a-c)}=\frac{1}{b-c}\ ;$

$3.\blacktriangleright \boxed {\ AK^2=\frac{bc}{a(b-c)^2}\cdot \sum a(a-b)(a-c)\ }\ \ \ ;\ \ \ 4.\blacktriangleright\tan \alpha=$ $\frac{\sin A-\sin B}{\cos A+\cos B-1}\ ,$ $\tan \beta =\frac{\sin A-\sin C}{\cos A+\cos C-1}\ .$

Proof of the proposed problem. Denote the circumradius $R_1$ of the triangle $BAK$ and the circumradius $R_2$ of the triangle $CAK\ .$

Using the relations $(1)$ and $(3)$ we obtain $:\ AK=2R_1\sin B=2R_2\sin C\Longrightarrow (R_1-R_2)^2=AK^2\cdot \frac{\sin B-\sin C)^2}{4\sin^2B\sin^2C}=$

$\frac{bc}{a(b-c)^2}\sum a(a-b)(a-c)\cdot \frac{R^2(b-c)^2}{b^2c^2}=$ $\frac{R^2}{abc}\sum a(a-b)(a-c)=$ $\frac{R^2}{4Rpr}\cdot 4pr(R-2r)=R(R-2r)=OI^2\ .$
This post has been edited 3 times. Last edited by Virgil Nicula, Nov 22, 2015, 12:22 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: 404396
  • Total comments: 37
Search Blog
a