Geometry – Name of Quadrilateral Shape Described by Two Radii and Two Arcs

Question

What is the name of the four sided shape described by two radii and two concentric arcs?
Like each black and white section taken individually on this image

Or the double score / triple score areas on a dart board.

I'm attempting to create each section individually, programmatically and need help on the best thing to google to get help actually making this.

Answer 1

Since I'm not a native English speaker, I was a bit reluctant to post something I made up myself as an answer. But Googling a little and the comment by amWhy encouraged me (showing me that I didn't really make it up, after all):

I think the word you're looking for is

Annular Sector

in analogy with a circular sector. I think that everyone knowing what a circular sector is, should be able to figure out what an annular sector is. An alternative word would be sector of an annulus.

You can specify an annular sector by giving its center $C$, the opening angle $\theta$ and the radii $0 \lt r \lt R$ of the two circular arcs bounding it:

Both these terms, annular sector and sector of an annulus give quite a few hits in Google. For example Googling for area annular sector brings up this link where you can find the formula

$$\textbf{Area of an angular sector:} \qquad \qquad A_{r,R,\theta} = \frac{(R^{2} - r^2)}{2}\cdot \theta$$

for the area $A_{r,R,\theta}$ of an annular sector with radii $0 \lt r \lt R$ and angle $\theta$ (measured in radians). This is because an annular sector is the circular sector of radius $R$ minus the circular sector of radius $r$ whose areas are $\dfrac{R^2}{2} \cdot \theta$ and $\dfrac{r^2}{2}\cdot\theta$, respectively. If the opening angle $\theta$ is given in degrees, you can convert it to radians using the formula $$\operatorname{radians} = \operatorname{degrees} \times \frac{\pi}{180}.$$

The length of the circumference $c_{r,R,\theta}$ of an annular sector with radii $0 \lt r \lt R$ and angle $\theta$ is

$$ \textbf{Circumference of an annular sector:} \qquad c_{r,R,\theta} = R \cdot (\theta + 2) + r \cdot (\theta-2)$$

where again $\theta$ is measured in radians. This is obtained by adding the lengths $R \theta$ and $r \theta$ of the two circular arcs and twice the length $(R-r)$ of the straight side.