How do you convert (x-h)^2+(y-k)^2=a^2(xh)2+(yk)2=a2 to polar form?

1 Answer
Oct 28, 2016

r^2-2rsqrt(h^2+k^2)sin(theta+phi_0)+h^2+k^2=a^2r22rh2+k2sin(θ+ϕ0)+h2+k2=a2
with
phi_0=arctan(h/k)ϕ0=arctan(hk)
or also

r = sqrt[h^2 + k^2] Sin( theta+phi_0) pm sqrt[ a^2 - h^2 - k^2 + (h^2 + k^2) sin(theta+phi_0)^2]r=h2+k2sin(θ+ϕ0)±a2h2k2+(h2+k2)sin(θ+ϕ0)2

Explanation:

The pass equations are

{(x=rcostheta),(y=rsintheta):}

then

(rcostheta-h)^2+(rsintheta-k)^2 =a^2 or

r^2-2rhcostheta-2rksintheta+h^2+k^2=a^2 or

r^2-2r(hcostheta+ksintheta)+h^2+k^2=a^2

Making now

h/k=tanphi_0 we have

r^2-2rk(tanphi_0costheta+sintheta)+h^2+k^2=a^2 or

r^2-2r(k/cosphi_0) (sinphi_0costheta+cosphi_0sintheta)+h^2+k^2= a^2

but cosphi_0=k/sqrt(h^2+k^2) so

r^2-2rsqrt(h^2+k^2)(sinphi_0costheta+cosphi_0sintheta)+h^2+k^2=a^2

Finally

r^2-2rsqrt(h^2+k^2)sin(theta+phi_0)+h^2+k^2=a^2 or also

r = sqrt[h^2 + k^2] Sin( theta+phi_0) pm sqrt[ a^2 - h^2 - k^2 + (h^2 + k^2) sin(theta+phi_0)^2]