平衡方程 ∑F=0 ∑Fy=0 ∑F=0 ∑M,(F=0 ∑M(F)=0 ∑M(F)=0          =  =  =  =  =  = ( ) 0 ( ) 0 ( ) 0 0 0 0 M F M F M F F F F z y x z y x 平衡方程：