# Complex Numbers

## Homework Statement

Find the equation of the locus of z where

$$\arg(\frac{z-2i}{z-3}) = \frac{\pi}{6}$$

## Homework Equations

$$\arg(x+iy) = \arctan(\frac{y}{x})$$

## The Attempt at a Solution

I wrote z=x+iy in which case we have

$$\frac{z-2i}{z-3} = \frac{x+iy-2i}{x-3+iy} = \frac{(x^3 - 3x + y^2 - 2y)+(6 - 2x - 3y)}{(x-3)^2+y^2}$$

I'm ommiting my working obviously, but i've checked it and i think it's correct. This means that

$$\arctan(\frac{6-2x-3y}{x^2 -3x + y^2 - 2y}) = \frac{\pi}{6}$$

$$18 - 6x - 9y = \sqrt{3} (x^2 - 3x + y^2 - 2y)$$

$$(x + \frac{2\sqrt{3} - 3}{2})^2 + (y + \frac{3\sqrt{3} - 2}{2})^2 = 13$$

which is the equation of a circle. This seems correct, but I think the locus is only part of the circle not the whole thing. How would you guys go about tackling this problem?

Thanks.

Last edited:

tiny-tim
Homework Helper
… which is the equation of a circle. This seems correct, but I think the locus is only part of the circle not the whole thing. How would you guys go about tackling this problem?

Hi kidsmoker! Personally, I'd use ordinary geometry.

Let A = 2i, B = 3, then the equation is angle BXA = 30º (taking care not to get -30º or 390º).

And the locus of points X with BXA constant is … ? And just check "by hand" the solutions you have to ignore because you get 390º. Okay so X lies on an arc of a circle passing through A and B (above to the right)?

tiny-tim
Homework Helper
Okay so X lies on an arc of a circle passing through A and B (above to the right)?

Yes! (erm … forget what I said about 390º … I forgot exactly what the question was. )

Cool thanks.

Yeah I was wondering what all that 390º stuff was about lol.