FM=2F, FN2--3F, FN3=5F, FN4-F 作轴力图F图 B =--:-: ----- Bl F 2F 5F F IIIIIIIIIIIIIIIIIIII 3F作轴力图FN 图 FN x 2F 3F 5F F A B C D FA FB FC FD O 2 , FN2= –3F， FN3= 5F， FN4= F  FN1 = F