令x²＋1=t,由于x²≥0,所以t=x²+1≥1;x²=t-1则原来的函数可转化为f(t)=3(t-1)+(6/t)=3t+(6/t)-3由均值不等式3t+(6/t)≥2√(3×6)=2√18=6√2当且仅当3t=6/t,即3t²=6,也就是t=√2时取等号,此...