How do you convert r(2 + costheta) = 1 into cartesian form?

2 Answers
Feb 8, 2017

(x+1/3)^2/(3/4)+y^2/(1/3)=1 See the markings at the center and the pole of the graph of this ellipse.

Explanation:

graph{(2sqrt(x^2+y^2)+x-1)(x^2+y^2-.0005)((x+1/3)^2+y^2-.0005)=0 [-1.25, 1.25, -0.625, 0.625]} The equation has the form

(1/2)/r=1+1/2costheta.

This represents the ellipse with focus S as pole, theta=0 for S-side

major axis, eccentricity e = 1/2 and semi major axis a = 2/3, from

a(1-e^2)=l=1/2.

The conversion formula is

r(costheta, sin theta)=(x, y).

So, 2r+rcostheta=2sqrt(x^2+y^2)+x=1 , giving

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

In the standard form, this is

(x+1/3)^2/(3/4)+y^2/(1/3)=1

Feb 8, 2017

3x^2+4y^2+2x=1

Explanation:

In converting from polar (r,theta) to Cartesian (x,y)
we have the relations:
color(white)("XXX")color(red)(r=sqrt(x^2+y^2))
and
color(white)("XXX")color(blue)(cos(theta)=x/sqrt(x^2+y^2))
(and others)

Polar form: color(red)(r)(2+color(blue)(cos(theta)))=1
becomes (in Cartesian form)
color(white)("XXX")color(red)(sqrt(x^2+y^2)) * (2 +color(blue)(x/sqrt(x^2+y^2)))=1

color(white)("XXX")rarr 2sqrt(x^2+y^2)+x=1

color(white)("XXX")rarr sqrt(x^2+y^2)=(1-x)/2

color(white)("XXX")rarr x^2+y^2=(1-2x+x^2)/4

color(white)("XXX")rarr 4x^2+4y^2=1-2x+x^2

color(white)("XXX")rarr 3x^2+4y^2+2x=1