F=∑F F=2F,F,=2F,F.=∑r i=l i=1 i=l 空间汇交力系的平衡 F=∑F=0 r.-0,立r,-0,∑r.-0 n i=l i=1 i=1R i F = F   1 1 1 , , n n n R x i x R y i y R z i z i i i F F F F F F = = = = = =    空间汇交力系的平衡 1 1 1 0, 0, 0 n n n i x i y i z i i i F F F = = =    = = = =  = 0 R i F F  