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