Polar rose

1. Nov 7, 2009

JanClaesen

I'm trying to express the polar rose as an implicit function:
r(t)=sin t

x = sin t * cos t
y = sin^2 t

Since sin t * cos t = (1/2) * sin 2t and sin^2 t = (1/2) * (1-cos 2t)

(2x)^2 + (1-2y)^2 = 1
4x^2 -4y + 4y^2 = 0

When I plot this, Maple plots a circle, where have I gone wrong?

2. Nov 7, 2009

arildno

"(2x)^2 + (1-2y)^2 = 1" This is an equation for a circle.

Your parametrization is not.

3. Nov 7, 2009

tiny-tim

Hi JanClaesen!

(have a theta: θ )

A rose is usually r = ksinθ or r = kcosθ … see http://en.wikipedia.org/wiki/Rose_(mathematics).

For k = 1, it is a circle.

(But you could have got the same equation if you'd just made it r2 = y )

4. Nov 7, 2009

JanClaesen

Do you have any hints on how to find the Cartesian equation for r(θ)=sin(2θ), I really can't seem to find it. :)

5. Nov 8, 2009

tiny-tim

Hint: multiply both sides by r2.

6. Nov 8, 2009

HallsofIvy

Staff Emeritus
And use the identity $sin(\theta)= 2sin(\theta)cos(\theta)$.

7. Nov 8, 2009

tiny-tim

He knows that
(and have a theta: θ )

8. Nov 8, 2009

JanClaesen

Wow, that was clever, thank you
For those interested:

xy = 0.5(x^2+y^2)(x^2+y^2)^(1/2)

(where x^2+y^2 = sin^2 (2θ) )

9. Nov 8, 2009

tiny-tim

(try using the X2 tag just above the Reply box )

That's it!

And then expand it , and put it all on the left:

(x2 + y2)3 - 2xy = 0.

10. Nov 8, 2009

JanClaesen

Yep, thanks again

Is there a human way to do this also for sin(3θ)? Or would that be a computer job?
I'm trying to do this now, but I have a feeling it's quite tough.

11. Nov 8, 2009

tiny-tim

Hint: try it for cos(3θ) + isin(3θ)

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?