788 Work and Energy △W,=F△rcos AW2= F2Arcos 0, △W.=F△rcos日 +)AW=F△rcos W=∑AW=∑ F Ar cos e7&8 Work and Energy 1 1 1 W = F r cos 2 2 2 W = F r cos  i i i W = F r cos  n n n +） W = F r cos i n i W =  W =1 i i n i F r cos 1 =  =