A.30° B.60° C.90° D.120°
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
A.70°~90° B.50°~70° C.30°~50° D.10°~12°