L=6ft, R=3ft (Diameter=6ft)
Let θ=ωt is the rider's angle around the center, starting from the line connecting the center and the camera, where ω is the constant rotation speed (not mentioned in the problem but assumed)
distance from the camera DC=sqrt( (L-Rcosθ)**2+(Rsinθ)**2) =sqrt( L**2-2LRcosθ +R**2)=sqrt( 36-36cosθ+9)=sqrt(45-36cos(ωt)
test 1, t=0, DC=sqrt(45-36cos0)=3, correct
test 2 , θ=ωt=180, DC=sqrt(45-36cos180)=sqrt81=9, correct