With reference to the figure at the side, you use the formula:
L2 = external_radium x tan(360°/2 x sections_number)
L1 = internal_radium x tan(360°/2 x sections_number)
A = external_radium - (internal_radium x cos(360°/2 x sections_number))
The following fields automatically compute the result. Beware that your browser has to support JavaScript.