94. Equations of angle bisectors (interior and exterior).

by Virgil Nicula, Aug 29, 2010, 2:39 AM

$\blacktriangleright$ Let $\left\|\begin{array}{c}
\Delta_1 (x,y)\equiv a_1x+b_1y+c_1=0\\\
\Delta_2 (x,y)\equiv a_2x+b_2y+c_2=0\end{array}\right\|$ be the equations of two lines which aren't perpendiculary, i.e. $a_1a_2+b_1b_2\ne 0$ .
Then the equation of the acute angle between these lines is $\frac {\Delta_1 (x,y)}{\sqrt {a_1^2+b_1^2}}+\frac {\Delta_2 (x,y)}{\sqrt {a_2^2+b_2^2}}\cdot \mathrm{sgn}\ (a_1a_2+b_1b_2)=0$ .

$\blacktriangleright$ Let $A_1A_2A_3$ be an equilateral triangle, where $A_k\left(x_k,y_k\right)$ , $k\in\overline{1,3}$ . Then the equation of interior $A_1$-bisector is $\left|\begin{array}{ccc}
x & y & 1\\\\
x_1 & y_1 & 1\\\\
x_2 & y_2 & 1\end{array}\right|\ +\ \left|\begin{array}{ccc}
x & y & 1\\\\
x_1 & y_1 & 1\\\\
x_3 & y_3 & 1\end{array}\right|\ =\ 0$ .

$\blacktriangleright$ Let $A_1A_2A_3$ be a triangle, where equations of the its sidelines are $\left\|\begin{array}{cc}
A_2A_3\ : & D_1(x,y)\equiv a_1x+b_1y+c_1=0\\\\
A_3A_1\ : & D_2(x,y)\equiv a_2x+b_2y+c_2=0\\\\
A_1A_2\ : & D_3(x,y)\equiv a_3x+b_3y+c_3=0\end{array}\right\|$ .

Then the equation of interior $A_1$-bisector is $\boxed {\ \mathrm{sign}\left|\begin{array}{cc}
a_1 & b_1\\\
a_2 & b_2\end{array}\right|\cdot\frac {D_2(x,y)}{\sqrt {a_2^2+b_2^2}}+\mathrm{sign}\left|\begin{array}{cc}
a_1 & b_1\\\
a_3 & b_3\end{array}\right|\cdot\frac {D_3(x,y)}{\sqrt {a_3^2+b_3^2}}=0\ }$ .


See the article "Distinctia analitica intre bisectoarele interioara si exterioara a unui unghi intr-un triunghi" from romanian review G.M.B., nr.3/1976.

Example. Let $ABC$ be a triangle for which the equations of its sidelines are $\left\|\begin{array}{cc}
AB\ : & 4x-3y-1=0\\\\
AC\ : & 3x+4y-7=0\\\\
BC\ : & 7x+y-33=0\end{array}\right\|$ . Then the equation

of interior $A$-bisector is $\mathrm{sign}\left|\begin{array}{cc}
7 & 1\\\
4 & -3\end{array}\right|\cdot\frac {4x-3y-1}{5}+\mathrm{sign}\left|\begin{array}{cc}
7 & 1\\\
3 & 4\end{array}\right|\cdot\frac {3x+4y-7}{5}=0$ , i.e. $x-7y+6=0$ .
This post has been edited 28 times. Last edited by Virgil Nicula, Nov 23, 2015, 1:52 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: 404395
  • Total comments: 37
Search Blog
a