a B XI BC段:M-Fx2+F(x2-a)=0 M= FA Y2-F(Y2-a a l2t+ aF a<x <l2 2 ( ) 0 M F x F x a − + − = A 2 a x l   BC段： L F x1 A B a C b x2 M = FA X2 - F(X2 - a) =- FX2 + aF a l