Find the inradius of triangle ABC.
Inradius = area / semi-perimeter =
[ 1/2 * a^2 * sqrt (3) /2 ] / [ 3a/2] =
[ sqrt (3) / 4 * a^2 ] / [ 3a/ 2 ] =
a / (2 sqrt 3)
sqrt (3) a / 6
