例2:已知l=12m,h=8mAm==0.04m 1By=0.0m,求4 解:构造虚设力状态 B材 2 94=-∑FRc1=+(12ha)=0.0075rad())解：构造虚设力状态 = − = − − − ) = 0.0075 rad 2 1 1 ( A Ri i By Bx l h  F c   ( ) FAy FAx 例 2：已知 l=12 m , h=8 m , Bx = 0.04 m By = 0.06 m , 求  A = ?