Help on theta region of polar double integral

269 Views Asked by At

I have a question about setting up the bounds on the theta integral of a polar double integral.

When you have a circle such as $(x-1)^2 + y^2 = 1$, the equation of the circle can be shown to be $2\cos(\theta)$. So $r$ will go from $0$ to $2\cos(\theta)$. Fair enough.

But the theta. I have watched one excellent online video (Prof. Leonard) who has the theta range of this integral as $0$ to $\pi$. On the other hand, the Stewart Calculus book has this same problem with a limit of $-\pi/2$ to $\pi/2$.

I know these may give the same answers, but which is the correct set up?

Thanks so much (and, yes, I've reviewed the other questions on the forum before posting this).

1

There are 1 best solutions below

8
On

As it pointed in Jyrki Lahtonen's comment if we consider $(x-1)^2 + y^2 = 1$, then polar coordinates gives $0 \leqslant r \leqslant 2 \cos \theta$. So we have region for $\theta$ from inequality $$0 \leqslant \cos \theta$$ If initially we choose $ \theta \in (-\pi, \pi]$, then we obtain solution $ \theta \in \left[-\frac{\pi}{2}, \frac{\pi}{2}\right]$.