# Relationship between two complex numbers

• Sep 29th 2010, 06:53 PM
arze
Relationship between two complex numbers
Given that $z=w^2$ where $z=x+yi$ and $w=u+vi$, prove that:
a) if u varies but v is constant, then the locus of $P(x,y)$ is a parabola,
b) if v varies and u is constant, then the locus of P is again a parabola.
Show that these two parabolas have the same axis and focus.

From the relation:
$x+yi=u^2-v^2+2uvi$
$x=u^2-v^2$
$y=2uv$
Which is the parametric equation for a parabola.
When u varies, the parabola has focus $(v,0)$ and axis $y=0$.
When v varies, the parabola has focus $(u,0)$ and axis $y=0$.

The parabolas face in opposite directions, and both cross the y-axis, assuming that the constants are greater than 0. My problem is how do I show that they have the same focus. They do have the same axis.
Thanks!
• Sep 30th 2010, 05:21 AM
HallsofIvy
Just to make it clear that "v is a constant", write c in place of v.

$x= u^2- v^2$ and $y= 2cu$.

From the second equation, $u= \frac{y}{2c}$

Put that into the first equation: $x= \frac{y^2}{4c^2}- c^2$ which is the equation of a parabola with vertex at $(-c^2, 0)$, axis along the x-axis, and opening to the right. You should have learned that the parabola $y= \frac{1}{4c}x^2$ has focus at (0, c). Adapt that to this problem.