Dx dy where R is the unit circle.

[squenshl]

Given the double integral $$\int\int_R$$ $$\sqrt{}x^2+y^2$$ dx dy where R is the unit circle.
We are only given the equation for the unit circle but don't we need more equations so I can change the equations to a single variable and then find the Jacobian so how do I find the Jacobian.
How do I find a point interior to R at which the Jacobian vanishes.

 

[nicksauce]

http://en.wikipedia.org/wiki/Polar_coordinate_system