the main condition can be found one of the following given methods:
if A = BCD, if we want to find the value of B, so B = A /CD, the similar method can be applied to find the value of C an D. These are C= A/ BD and D = A /CB
in our case we have C = 2Pi R,
solve for R does mean, searching the value of R, so
if we want to find the value of R,
it is R = C / 2Pi
