x = lal/(b+c)+lbl/(a+c)+lcl/(a+b) = lal/(-a)+lbl/(-b)+lcl/(-c) = -(lal/a+lbl/b+lcl/c) ；已知,有理数a,b,c均不为0,且 a+b+c = 0 ,可得：a,b,c为一正两负或,两正一负；分情况讨论：① 当a,b,c为一正两负时,lal/a...