F=f+F F RI FR2=F2+FR3=∑F F+F R1-11 F R2 ∑ F F 力多边形 FR=Fa+F=∑F=∑FFR 1 F1 F2    = + = = + = 31 2 1 3 i FR FR FR Fi     = + = = = − i ni FR FRn Fn Fi F      1 1 ... ... ... = = + = 31 2 1 3 i FR FR FR Fi     FR1 F1 F2    = + 力多边形