解给虚位移0,δS ∑6Wn=-F2s+2Fg=0 δq与δs满足如下关系: S 8S 2丌h fh ∑6W=2F7- l6=0 2兀 因δ是任意的,故 2F-=0F.、4z fh F 2丌 h 冈心心解:给虚位移 , s, W = −FN  s + 2Fl = 0 F 与 s 满足如下关系： h  s   = 2   =      = − 0 2 2    F h W Fl N F 因是任意的 ，故 0 2 F h 2Fl N − =  F h l FN 4 =