Let $T(z)=1/z$. Find the image of $y=2x+1$ under $T$.

I assume x and y are the real and imaginary part of z.

Basically what I need was let $w=1/(x+(2x+1)i)$ then multiplied by the complex conjugate we have that the real imaginary part of x.

We have


Then should we equate coefficient with the equation of a circle? I dont know how to proceed.

From university, I know that the image must be a circle or a line since the transformation is a Mobius transformation. How is an A level student meant to get this?

i just looked at another question. Do i need to find two points such that this line is the locus for those two points? Then i sub that in?

  • $\begingroup$ Did I edit your post correctly? $\endgroup$ – Arbuja Apr 29 '17 at 20:38

Let $w=\frac 1z\implies z=\frac 1w$

We write $w=u+iv$, so that $$z=x+iy=\frac{1}{u+iv}=\frac{u-iv}{u^2+v^2}$$

Then $$y=2x+1\implies -\frac{v}{u^2+v^2}=\frac{2u}{u^2+v^2}+1$$

This is a circle $$u^2+v^2+2u+v=0$$

  • $\begingroup$ Nicely done :). $\endgroup$ – Faraad Armwood Apr 29 '17 at 20:54
  • $\begingroup$ [+1] You can proceed by equivalence, i.e., replace the $\implies$ arrow by an $\iff$... $\endgroup$ – Jean Marie Apr 29 '17 at 21:06
  • $\begingroup$ Urgh i c. Express in term u v before substituting... $\endgroup$ – Lost1 Apr 29 '17 at 21:20

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