A.51.7 B.80 C.105 D.123
A.F=(π/2)d2P B.F=(π/3)d2P C.F=(π/4)d2P D.F=(π/5)d2P
A.F=π/40(D2-d2)P B.F=π/4(D-d)2P C.F=π/4(D+d)2P D.F=π/4(D2+d2)P