Find appropriate parametrization to find area bounded by a curve

wifi
Messages
115
Reaction score
1
Problem:

Use an appropraite parametrization x=f(r,\theta), y=g(r,\theta) and the corresponding Jacobian such that dx \ dy \ =|J| dr \ d\theta to find the area bounded by the curve x^{2/5}+y^{2/5}=a^{2/5}

Attempt at a Solution:

I'm not really sure how to find the parametrization. Once I have that, calculating the Jacobian is simple. Then all that's left is computing a double integral. Right?
 
Physics news on Phys.org
wifi said:
Problem:

Use an appropraite parametrization x=f(r,\theta), y=g(r,\theta) and the corresponding Jacobian such that dx \ dy \ =|J| dr \ d\theta to find the area bounded by the curve x^{2/5}+y^{2/5}=a^{2/5}

Attempt at a Solution:

I'm not really sure how to find the parametrization. Once I have that, calculating the Jacobian is simple. Then all that's left is computing a double integral. Right?

Hint: Your parametrization hints at polar co-ordinates.

##x = rcos\theta, y = rsin\theta##
 
But if I use ##x=rcos\theta## and ##y=rsin\theta##, then I have ##dx=rd\theta## and ##dy=rd\theta##. Also ##|J|=r##. However we want ##dx \ dy=|J|dr \ d\theta##.

Using ##x=rcos\theta## and ##y=rsin\theta##, then the curve is given by (rcos\theta)^{\frac{2}{5}}+(rsin\theta)^{\frac{2}{5}}=a^{\frac{2}{5}}

So to find the area bounded by this curve we want to perform a double integral. I'm just confused on setting it up. Now that we're in terms of ##r## and ##\theta##, how do we figure out the limits of integration for the given curve?
 
Last edited:
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
Back
Top