Magnetic field at O due to segments FA and CD is equal to zero.
Magnetic field at O due to semi-circle ABC:12μ0I2R2(−^k)
Magnetic field at O due to semi-circle DEF:12μ0I2R1(−^k)
→Bnet=12μ0I2R2(−^k)+12μ0I2R1(−^k)=μ0I4(1R1+1R2)(−^k)=μ0Iπ4π(1R1+1R2)(−^k)where ^k is out of the plane of the loop and perpendicular to the loop