TITLE: Ноw many equal circles can be placed around a circle?
QUESTION [9 upvotes]: How many circles of radius $r$ can be placed around a circle of radius $R$ (close to it)? $r$ can be bigger, equal or smaller than $R$.

REPLY [9 votes]: Draw the two lines through the center of the central circle that are tangent to one of the touching circles.  Call the angle between them $\theta$.  The question then is how many times will $\theta$ go into the full circle?  You need $\left\lfloor\dfrac{2\pi}{\theta}\right\rfloor$, or if you're using degrees,  $\left\lfloor\dfrac{360}{\theta}\right\rfloor$.
Now draw the line through the center of the two circles.  The angle between that line and one of the lines you drew earlier is $\theta/2$.  The distance between the centers is $R+r$.  Now draw the segment from the center of the touching circle to the tangent line.  Its length is $r$.  Now you have a right triangle in which the hypotenuse has length $R+r$ and one of the legs has length $r$ and the angle opposite that leg is $\theta/2$.  Therefore
$$
\sin\frac\theta2 = \frac{r}{R+r}.
$$
Take arcsines and multiply by $2$ and you've got $\theta$.