3.画弯矩图 a b A~1 B 0≤x1≤a X1 0时,M X2 06 Xu =a时 a ab M=-CFl2taF a<x2< ab x2=a时,M 直线 x2=l时,M=0F x1 A B a C b x2 3.画弯矩图 M =- FX2 +aF a l 2 a x l   M = 0 ab M F l = ab M F l = M = 0 1 x = 0 时， 1 x a = 时， 2 x a = 时， 2 x l = 时， A 1 1 b M F x Fx l = = 1 0   x a 1 0   x a ab M F l = 直线