A circle has circumference formula C=2pirC = 2\\pi rC=2pir. Solve for rrr in terms of CCC.
r=fracC2pir = \\frac{C}{2\\pi}r=fracC2pi
r=2piCr = 2\\pi Cr=2piC
r=C−2pir = C - 2\\pir=C−2pi
r=frac2piCr = \\frac{2\\pi}{C}r=frac2piC