F±F2=(a1+jb,)±(a2+jb2) =(a1±a2)j(b,±b2) FF=Fei 2=FF(+2) F1F∠θ,F 1∠0.-9 F,F,∠0,F 也可用平行四边形法则 f,+F1 2 2 1 2 2 1 1 2 1 j( 1 2 ) 1 2 j 2 2 j 1 1 2 1 1 2 1 2 1 2 1 1 2 2 F F F F F F F F F e F e F F e (a a ) j(b b ) F F (a jb ) (a jb ) =  −    = = = =  +   = +  +    + 也可用平行四边形法则 F=F1+F2 F1 F2