Elementary Geometry for College Students (7th Edition)

We first use trigonometry to find the missing sides: $AC = 12sin(60) = 10.4$ $BC = 12 cos(60) = 6$ We use the known formula to obtain: $r = \frac{ab}{a+b+c} \\ r = \frac{6(10.4)}{6+10.4+12} \\ r = 2.19$