解由题设及余弦定理可得cosB=(a²+c²-b²)/(2ac) =[(a²+c²)/(2ac)]-(1/2)即cosB=[(a²+c²)/(2ac)]-(1/2)由基本不等式可得 a²+c²≥2ac＞0∴(a²+c²)/(2a...=[(a²+c²)/(2ac)]-(1/2)即cosB=[(a²+c²)/(2ac)]-(1/2)——————这步不太懂-------------我看懂了