因為間距相等
設圓心角為θ,
⌒ab=5π=2OA*θ
⌒cd=4.5π=2OC*θ
OE = OC - (OA - OC) = 2OC - OA
⌒ef=2OE*π= 2⌒cd - ⌒ab = 9π - 5π = 4π