9(x)= O= 2 q(x) d xd=q)×dkxx FR =qL d 31d-59d-9 2 3FR = qL2 L d = d q(x) dx x x l q q(x) =   =   l Q d q x dx x 0 ( ) l q Q 2 = 2 0 2 2 3 l q x dx l q l d q l   = = d l 3 2 =