Question

An equilateral triangle of side 9 cm is inscribed in a circle find the radius of the circle.

Answer 1

9^2 = r^2 + r^2 - 2*r*r*cos(120) 
81 = 2r^2 - 2r^2*(-0.5) 
81 = 2r^2 + r^2 
3r^2 = 81 
r^2 = 81/3 
r^2 = 27 
r = sqrt(27) 
r = 3*sqrt(3) 
r =~ 5.1961524227066318805823390245176