Find the Cartesian equation of the locus of arg((z-4)/(z+4))=pi/4?

Thanks!

2 Answers
Nov 29, 2017

This equation is circle with centre (-4, 0) and its radius r=4sqrt2

Explanation:

Set z=x+yi, so arg((z-4)/(z+4))=pi/4

arctan((z-4)/(z+4))=pi/4

arctan((x+yi-4)/(x+yi+4))=pi/4

arctan((y-4)/x)-arctan((y+4)/x)=pi/4

After taking tangent both sides,

[(y-4)/x-(y+4)/x]/[1+(y-4)/x*(y+4)/x]=tan(pi/4)

(-8/x)/[(x^2+y^2-16)/x^2]=1

-8/x=(x^2+y^2-16)/x^2

x^2+y^2-16=-8x

x^2+8x+y^2-16=0

x^2+8x+16+y^2=32

(x+4)^2+y^2=32

This equation describes a circle with its centre M (-4,0) and its radius r=4sqrt2.

Nov 29, 2017

x^2+(y-4)^2 = 32

Explanation:

We know that

z-4 = r_1 e^(i phi_1) with phi_1 = arctan(y/(x-4))
z+4 = r_2 e^(i phi_2) with phi_2 = arctan(y/(x+4))

then

"arg"((z-4)/(z+4)) = "arg"(r_1/r_2e^(i(phi_1-phi_2))) = phi_1-phi_2 = pi/4 or

arctan(y/(x-4))-arctan(y/(x+4))=pi/4

now

tan(arctan(y/(x-4))-arctan(y/(x+4))) = tan(pi/4) = 1 or

(8 y)/(x^2 + y^2-16) = 1 or

x^2+y^2-8y-16=0 or

x^2+(y-4)^2 = 32

NOTE

From tan(phi_1-phi_2) = (tanphi_1-tanphi_2)/(1+tanphi_1 tanphi_2)

making

phi_1 = arctan(y/(x-4)) and phi_2 = arctan(y/(x+4))

and knowing that tan(arctan xi))= xi we have

((y/(x-4))-(y/(x+4)))/(1+(y/(x-4))(y/(x+4))) = (8 y)/(x^2 + y^2-16)