0=(y-xdL-Ldx+dldx (y-x)dL= Ldx dx 积分 dx L x2 1-x F dx In=xyx 2021/2/21 上页下及2021/2/21 0 = (y − x)dL − Ldx + dLdx (y − x)dL = Ldx y x dx L dL −  = 积分   − = F W x x y x dx L dL 1 2  − = 1 2 ln x x y x dx W F