如下图所示，这题用换元法
(a-c)[1/(a-b)+1/(b-c)]=[(a-b)+(b-c)][1/(a-b)+1/(b-c)]=2+[(b-c)/(a-b)+(a-b)/(b-c)]≥4,∴4≤(a-c)[1/(a-b)+1/(b-c)]
