=(a－5c)²＋a²＋1/(ab)＋1/[a(a－b)]=(a－5c)²＋a(a－b)＋ab＋1/(ab)＋1/[a(a－b)]=(a－5c)²＋{a(a－b)＋1/[a(a－b)]}＋[ab＋1/(ab)]≥(a－5c)²＋2＋2≥4.其中等号取得的条件是a(a－b)=1/[...