179. The range of |z| , |z^2+a|=|bz+c|. Particular cases.

by Virgil Nicula, Nov 28, 2010, 9:37 AM

Proposed problem. Ascertain the range of $|z|$ , where $z\in\mathbb C$ for which $\left|z^2+a\right|=|bz+c|$ and $\{a,b,c\}\subset\mathbb R$ are given , $a>0$ . Study only some particular cases.

Proof of the following particular cases. Denote $|z|=\rho$ and $s=z+\overline z$ . Observe that $z\cdot\overline z=\rho^2$ and $s=z+\overline z=2\cdot \mathrm{Re}\ (z)$ . Therefore :

$\blacktriangleright\ \left|z^2+1\right|=|z-1|$ $\iff$ $\left(z^2+1\right)\cdot\left(\overline z^2+1\right)=(z-1)\cdot \left(\overline z-1\right)$ $\iff$ $\rho^4+s^2-2\rho^2+1=\rho^2-s+1$ $\iff$

$\left(\rho^2-\frac 32\right)^2+\left(s+\frac 12\right)^2=\frac 52$ . In conclusion, $\left|\rho^2-\frac 32\right|\le \sqrt {\frac 52}$ $\iff$ $|z|\le\sqrt {\frac {3+\sqrt{10}}{2}}$ with equality if and only if $\mathrm{Re}\ (z)=-\frac 14$ .

$\blacktriangleright\ \left|z^2+1\right|=|4z+3|$ $\iff$ $\left(z^2+1\right)\cdot\left(\overline z^2+1\right)=(4z+3)\cdot \left(4\overline z+3\right)$ $\iff$ $\rho^4+s^2-2\rho^2+1=16\rho^2-12s+9$ $\iff$

$\left(\rho^2-9\right)^2+(s-6)^2=125$ . In conclusion, $\left|\rho^2-9\right|\le 5\sqrt 5$ $\iff$ $|z|\le\sqrt {9+5\sqrt 5}$ with equality if and only if $\mathrm{Re}\ (z)=3$ .

$\blacktriangleright\ \left|z+\frac 1z\right|=a>0\ \ \implies\ \ \frac {-2+\sqrt {a^2+4}}{2}\le |z|\le\frac {2+\sqrt {a^2+4}}{2}$ with equality if and only if $\mathrm{Re}\ (z)=0$ .

$\blacktriangleright\ \left|z^2+1\right|=2\cdot |z+1|$ $\implies$ $|z|\le \sqrt 7$ with equality if and only if $\mathrm{Re}\ (z)=1$ .


An easy extension. Consider $\{a,b\}\subset\mathbb R$ and $z\in \mathbb C$ for which $\left|z^2+1\right|=2\cdot |az+b|$ . Denote $A>0$

so that $A^2=\left(a^2+1\right)\cdot \left(a^2+b^2\right)$ . Prove that $|\mathrm{Re}\ z-ab|\le A$ and $\left|\left(2a^2+1\right)-|z|^2\right|\le 2A$ .
This post has been edited 20 times. Last edited by Virgil Nicula, Dec 1, 2015, 11:13 AM

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