1/a²+1/b²+ab≥2√21/a²+1/b²+1/ab=(1/a²+1/b²+2/ab-1/ab)=(1/a-1/b)²+1/ab1/a²+1/b²+ab≥1/a²+1/b²+1/ab=(1/a-1/b)²+1/ab≥[(1/a-1/b)/8]²×2√...