Question 131351
There are {{{2*pi}}} radians in a circle.
The circumference, {{{C}}} of a circle is {{{C = pi*d}}} where
{{{d}}} is the diameter
The fraction of the circumference of an arc which is {{{d}}} in
length is {{{d/(pi*d) = 1/pi}}}
The fraction of {{{2*pi}}} radians of this arc is
{{{(1/pi)*2*pi = 2}}}radians
The measure, in radians, of the central angle  = {{{2}}}